Erika, I understand the worry and fear well. I won't go into details, but let's just say Ivy and I understand very very well and we have our own horror stories to tell.

And I will say this: You still have a chance to keep everything stable and everybody sane. I made a commitment to Ivy, and I will not give her up as I cannot rehome her in good conscience. She has serious issues and it would be downright irresponsible for me to release her into new hands. So, in a sense, I am obligated to keep her. But you are not by any means obligated to keep Jemima, even as a foster. I know it sounds cruel, but I know what it's like to own an unstable dog in a multi-dog home and I know what it entails and I don't want you to walk into that.

If Jemima is truly dog aggressive (and I would make multiple, full assessments and enact NILIF and similar discipline regiments before deciding for sure), I would consider two things:
1) Exactly how unstable is she and to what extent and in what situations?
2) Knowing #1, can you, in good conscience, rehome her? And what type of home would she need and what quality of life would she receive with those stipulations?

But before you decide: Try to consider and understand her triggers. She sounds very attached to you. If you were with them when she attacked, it's very likely that she was fighting over you. It doesn't mean she's "dominant" or all that other fudge. It just means that she doesn't have self control, she doesn't have proper socialization skills, and she desperately needs guidance. It's workable, but Jemima may always carry an inherent risk if this is deeply biological. And it very well could be. I know this all too well, and I give you my deepest sympathies.

Sleep on it. And if you need tips on how to control these types of dogs, don't hesitate to PM me. I'm all too familiar with these situations...