Sharing Account Inventories

I don't know if there's a way to do this or it's been brought up before, but I recently got my sister into Magic and she's been using my cards. As she starts growing her own collection, she'll be adding them to her own profile, but is still able to build decks and such using mine, so it would be nice if she could see my collection and put cards from it into her decks without being on my profile.

So what I'm hoping for is:
- The ability to link accounts to share inventory.
- The ability to maintain ownership of said inventory, so that the linked account couldn't accidentally trade something that wasn't theirs or a similar situation.
- Shared cards maintaining a correct deck count across accounts. If I've got three of a card in a deck and she's got three of the same card in her deck, it should show that six are being used (unless she has some as well, in which case, it would subtract however many she has from that total. Or not. That might over-complicate things.)

I don't know how plausible something like that is, but I know there are plenty of other people in my situation who might benefit from it.