I know the temptation is horrible but the most you can do is just ask her sister to back off of any stressful situations she might place upon your friend...that she's going through a bit of a rough spell at the moment. Your friend will tell everyone when she comes to grips with it.

Grant me the senility to forget the people I never liked,the good fortune to run into the ones I do,and the eyesight to tell the difference.