its a real tough one!

yeah he's fcuked your sister about and i do agree that blood is thicker than water, but at the end of the day you have built up a friendship with this guy and he has helped you out at times when youve needed it!

If i were you id make it very clear that you do not agree with what hes done. If your sister doesnt mind you staying in touch with him, then i dont see why you shouldnt if you want to. He hasnt done anything personally wrong to YOU!

but im sure he knows that you aint best pleased with the way hes treated your sister.