my sister is having improper relationship with one of my friend...in front of me she is calling my friend as brother and they both behave like this...but after some time one of my other friend bring to my notice about their relationship and i even happened to see  them in various places in ...i asked her about this she is telling don't think like that he is like her brother only....now am in very critical position don't know what to do....how to proceed....how shall i tell her that what she is doing is wrong...........