Soul Sisters is often thought of as a mono-white aggro deck with lifegain components, but I view it more as a midrange deck. Whereas with aggro decks, you want to kill your opponents quickly, Soul Sisters can play the long game just fine. Firstly, I will introduce the key cards.
Soul Warden/Soul's Attendant: The namesake of the deck. These one-drops net you one life whenever a creature enters the battlefield, be it yours or your opponents. These cards are contribute to a lot more than simple lifegain. Rather than playing a deck of just "good stuff", Soul Sisters creates synergy and generates advantages with the Soul engine.
Auriok Champion: This card is awesome. Sure, she's simply a 1/1 Soul Sister for 2 mana, but she has protection from Black and Red. This means she will ideally stay on the board for a long time. No Bolt, Terminate, Decay, or even Anger of the Gods can stop her. Your opponent's few answers to this card are Cryptic Command or Path. Note: Don't attack with her against 2 mana up including blue. Their Snapcaster will block it, and it is not worth it.
Ajani's Pridemate: Your Tarmogoyf. Ajani's Pridemate is your two-drop beater. Drop him turn 2 with turn 1 Soul Sister and he's a 3/3. Then, whenever an individual creature enters the battlefield, he grows and grows. It's not uncommon for him to become a double digit giant. He can threaten the opponent fast and tussle with most any creature. Expect him to get Terminated, Decayed, Bolted, or Pathed often. He is scary when he gets going.
Serra Ascendant: A simple 1/1 with lifelink as a benefit. This creature shines when your strategy and engine have been in your favor enough to get you above 30 life. Once there, Serra Ascendant is hard to ignore, especially if even one swing gets through.
Ranger of Eos: Ranger is awesome against the attrition decks. He will essentially draw you 2 creatures that you need. Between 8 Soul Sisters, 3-4 Serra Ascendants, and possibly a Martyr of Sands, he can adapt to your liking and shape the mid-late game how you see fit.
Archangel of Thune: She is the bomb of your deck. For every Soul Sister present, she will pump your team by that many Sisters and gain you that much life. For example, you have Soul Warden, Soul's Attendant, and Auriok Champion. Archangel enters. Assuming no disruption occurs, you will gain 3 life, and put 3 counters on everyone on your field INCLUDING the Archangel herself because of how the triggers resolve. She will turn games around in as quickly as the turn she comes out.
Spectral Procession: You get 3 evasive dudes. They can chump. They can pressure. The will most likely gain you life. With 1 Sister, that's 3 life. With 2, that's 6 life. You can make some big gains with this card. Also, it doesn't trigger Eidolon and they can't IoK it.
Path to Exile The deck's premier removal. Path is self-explanatory. I've even pathed my own creatures (usually a token) to get that 4th or 5th mana for Ranger/Angel. #nextlevelplays /s As always, be wary about how much mana you're giving your opponent given the game's circumstances.
Brave the Elements: For one mana, you can win a combat trick, survive an attack to save you a turn, push with evasion for lethal, counter removal and boardwipes like Bolts, Decay, Terminate, Path, Dismember, Angers, and Pyroclasms. The versatility of this card is fantastic in a format as diverse as modern especially when opponents observe your wide, creature-heavy strategy and want to bring in the Angers/Pyroclasms/Whipflares.
Windbrisk Heights: This card sneaks in your Archangel to destroy your opponent, or generates advantage by casting Ranger or Procession. The former is the most ideal, but even a Brave the Elements or Path to Exile can be a great pre-block flash under a Windbrisk. Soul Sisters makes lots of creatures so activating is not relatively hard.
Flagstones of Trokair: Honestly, this card is great for deck thinning or shuffling as you need. If you don't wanna spend the money, 4 plains will work.
Ghost Quarter: Helps against Tron, one of Soul Sister's worse matchups. Also can be used in conjunction with Flagstones to break even with 2 white should you need all white mana. I run one because between the Sisters, Auriok Champion, Path, Brave, and Spectral Procession, I find myself needing actual white mana often.
Plains: Add white to your mana pool.
Now that I've talked about core cards, I'll explain some of the nuances that make builds different.
Martyr of Sands: I like Martyr, but I don't love her. Her on turn 1 with turn 2 activation and Serra Ascendant feels like cheating sometimes, but if you don't draw her early game, she's pretty underwhelming since your strategy most likely involves flooding the board meaning less cards in hand. That said, she is a great one-of (which is how may I personally run) because Ranger can tutor her should you need some life to save your bum or get Serra Ascendant online.
Thalia, Guardian of Thraben: Thalia is amazing because your deck is not particularly good against the dedicated blue decks or creature-lite control decks. I personally run 1 in main and 3 in side. There are occasions where she hurts your instants/sorceries, but being a predominantly creature-based deck, Thalia has more synergy than harm.
Secure the Wastes: This new, little card is another nice one-of. Secure is like Processions but instant speed so when you have lots of mana, you can flash this in for all your available mana and then have some attackers/blockers, or if you just need to trigger the Sisters for your Pridemate/Ascendant to get bigger. Usually a great late-game top-deck. Sadly, you cannot Windbrisk this card.
Next I will talk about why I believe some cards usually associated with Soul Sisters aren't as great as people think.
Squadron Hawk: Very underwhelming in my opinion. Drawing multiples is disgustingly bad and at the end of the day, you get a 1/1 with flying for 2 mana. People note his synergy with Martyr, but like I mentioned, even running multiple Martys isn't a smart plan. While gaining life is fantastic, actively achieving that goal leads you with a board state that has something to be desired. i.e. sacrificing Martyrs and playing Hawks will only get you so far. Using the Sisters as a passive and more synergistic strategy will better you as they contribute a lot more than straight-up life gain.
Honor of the Pure: I see the argument for this card. It develops the aggro strategy better, but like I mentioned at the beginning of this post, Soul Sisters can do some serious work even as a midrange deck. If you're looking to go more aggressive, by all means Honor is fantastic, but Honor's job can be done in a better fashion with Archangel should you choose to play the longer game. Seriously, Soul Sisters has lasting power and it's very rare to die early.
Before introducing Sideboard Cards, I wanna talk about the deck's matchups. Soul Sisters has game with just about any creature deck. Naya Zoo, Bant Zoo, Affinity, Jund, Abzan, you name it. This is a no brainer. If your opponent plans to kill you with creatures, plan on slowing down their plan with every new creature played because of the Soul Sister engine. Even Delver is an okay matchup because Angler/Tasigur can't beat Auriok Champion, and Young Pyro will trigger any Sister available. Where Soul Sisters lacks is against control decks and creature-lite decks. Decks like Scapeshift, Grixis Control, and Blue-based Tron decks are not easy. With no creatures on your opponent's side, your strategy is only half as effective. There is a special note against Twin and Abzan Coco. If they go infinite, so do you. For every Exarch they make, you gain that much life for every Soul Sister in play. Be careful against Pestermite as you will need 2+ Sisters. This is why Auriok Champion is super important against Twin. They cannot remove it and their combo is hosed. Against Abzan Coco, things get so much better if you have an Archangel. Granted, their going infinite is not as easy to counter as the regular old Twin. I will mention some cards to help out this match up momentarily. Lastly, Infect and Tron are probably your worst matchups as Infect ignores your lifegain and Tron doesn't play enough creatures to facilitate your plan. Plus, Karn and Ugin are hard to beat. You can still Path Wurmcoil.
In the sideboard, one of the most important cards to help against bad matchups is
Suppression Field: This cards is godly and is an amazing answer to so many bad matchups. Whereas Stony Silence is pure Affinity hate (and a bit of Tron), Suppression Field hits so many other decks. Fetchlands against most decks, Affinity's creatures, Planeswalkers like Lili, Jace, or Karn, Ooze, Olivia, Tasigur, Loothouse, Living End (Cycling), Grishoalbrand, Ad Nauseum, Twin, Inkmoth Nexus, Pendlehaven, Viscera Seer, you name it. Suppression Field is so strong against so many decks. It has won me games against my harder matchups like Tron, Grixis Control, and yes even Infect. It absolutely kills Living End and Ad Nauseum as I mentioned and makes Grishoalbrand's life even harder because their Griselbrand and Borborygmos can't kill you as effectively. Suppresion Field is especially important against Tron because their Karn and Maps will be depleted in terms of efficiency. The only nonbo this has with your deck is Martyr, Ghost Quarter, and Windbrisk. Be wary of that when developing your strategy/keeping your opening hand.
Kataki, War's Wage: He's great against Affinity and also Tron. I just prefer him to Stony Silence because he's a body, triggers the sisters, and Suppression Field basically does what Stony Silence does but better.
Thalia, Guardian of Thraben I mentioned this earlier, but she's just the best against your worse matchups. Against Infect, Grishoalbrand, or blue-decks like Delver, Thalia just wrecks. Also good against Burn if their Skullcracks and Atarka's Command have been giving you a hard time.
Mark of Asylum: This is good against Anger, Pyroclasm, and Whipflare or even Bolt. Gives Jund, and Grixis a harder time. Or even Tron since they usually main Pyroclasm.
Rest in Peace: For the most part, RIP is good as your strategy rarely involves the graveyard. RIP is good against Snapcaster decks and graveyard combo decks. It's also good against Goyf/Tasigur, but for the most part, creatures are not what you are worried about when playing Soul Sisters.
Leyline of Sanctity: Another great shield. Stops combos like Ad Nauseum or Storm, but it really shines against Jund. Between, Lili, IoK, Thoughtseize, and sometimes Bolt, Leyline is sweet if you open it. If not, you are playing white, and the game will most likely go long because your life will constantly fluctuate so there is still value in hard-casting Leyline.
Hallowed Moonlight: What I like is this cards counters cheat-y cards like Grishoalbrand or Unburial Rites. What I don't like is that your opponent can almost always "fail to find". It still draws you card and can even prevent Twin if you have no Sister. This card is also great against Collected Company, namely Abzan Coco because they will be Coco-ing and Chording so often.
Sundering Growth: As a deck with various token elements, this is our choice of artifact/enchantment removal. Torpor Orb and Leyline of Punishment suck, so this card helps against that. Also takes out the accursed Inkmoth Nexus because we hate dying to Infect!
Return to the Ranks: Of all the weird mass revival, I like this best because it can save a lot of the Sisters and Pridemates. Best of all, they all trigger upon entering! Return helps make a comeback if Brave the Elements and Mark of Asylum weren't enough to save you from Pyroclasm, Whipflare, Damnation, or Engineered Explosives. It is a nonbo with RIP though, so watch out for that. Both are cards I want to sideboard in against Grixis/Jund, but in the wrong sequence kinda suck together.
So there you have it. These are my opinions about the deck. Please let me know what you all think especially if I missed any tips to give. Again, I cannot stress enough how you ought to first and foremost be a midrange deck attempting to play out the game until you get to Archangel/online Serra Ascendant. Yet, the deck is still fully capable of racing decks like Infect should the situation call for it. After all, you'll be playing lots of bodies to deal damage and have the lasting power to survive lots of hits and removal spells.