Not an easy answer but we go through that here as well. My stepson's sister also lives with us and is not very understanding although we are careful to try to spend as much time with her as we do with my stepson. Careful discussion works for a day or two and then she reverts to the "why me?" and deliberate instigations because she is just so fed up. Underneath it all though she is deeply concerned for her brother (especially since we are moving on to Youth Protection). The only thing I can say that works here is constant reminders (and I do mean constant) and also anticipation on the parents' part of any situation that could possibly become negative or even violent (yup - they do go at it) - then we move to distract them or plan an activity that separates them but is still positive.
Also, planning free time where the siblings are separated but at least with one of the parents has really worked. Even the "normal" kids need to feel that positive glow of undivided parental attention. Nearly impossible, but doable. Unfortunately, we've come to the conclusion in our family that separating them temporarily does help them to cope when they are actually together. Could be an evening, a day, an afternoon or a sleepover at Grandma's.
I know exactly how you feel!
