Couple of things:
1) maybe since the other cats are moving out of kitten hood, she sees them more as a threat.
2) she might be in some kind of pain. The weight loss is a flag for me. Even with more activity, you might want to get her checked out.
We at one time had 4 cats, who pretty much hated one another but tolerated the other. I don't think you'll need to re-home. They'll work it out amongst themselves.