we had a similiar situation this year. we ended up asking the two to a mediation with me(as alum) and the advisor. It helped a bit but not much. I could easily have laid blame to one sister than the other for their problems, but it was just who she is, her personality, so there wasn't much we could do but ask them both to try and tolerate each other more.
Howver, in your case maybe a mediation would help...sometimes J-board can help with this, but if you want to keep your sisters out of it ask your advisor or another alum to sit down with the two of you
(and remember to use "I" statements
)
LITP