Why cant my sister talk to me? How can I reach her?

My younger sister just broke up with her girlfriend,yes shes a lesbian. I think. Rumors said because of third party. The ex-girlfriend is nice to me an very kind and I really like her. Im trying to convinvce my sister to give her ex a chance. The ex girlfriend really lover her, I could tell. However, my sister seems to not listen to me, and keep telling that ler her alone. Shes always taking the fault, saying 'yes Im the bad guy here. Please help me. What can I do?