All I can say is give her time.
If you can reassure her that any changes will be gradual, and that you would value her help in transition, that could help her too. Have you spoken to your sisters yet? They could help a lot, if they are supportive. One important thing I would suggest is to reassure her that it is not her fault in any way. It just happens.
She has just had the rug pulled out from under her in a big way, and will take a while to come to terms with it. Don't feel guilty though, it would have come out sooner or later, and possibly in far less pleasant circumstances.
There will be the usual grieving process, denial, sadness, anger, guilt etc. Just be there for her too, and she should come round in time. Seeing she works at a hospital, maybe she can get some counselling through that, to help understand your situation and her reaction.
As a parent who has a FtM son,( who transitioned several years before I started this process,) I can see what she is experiencing. Please tell her that if she wants to PM me through here I am more than happy to talk about her concerns.
Karen.