Scenario.
I got to know my My friend Mr "A"  through my sister, and  he was dating my  sister's best friend (also my friend). My sister got to know her best friend through me. The point is that my sister's friend is overly obsessive with this dude who seems to have less affection for her. Now my  friend Mr "A" is somehow interested in my sister and my younger sister is not telling me anything and she thinks that I do not know of their moves. Mr friend Mr A is also keeping me in the dark.

My interest here is that I cannot lose my friendship with my sister's  girlfriend because she has so much helped us a lot   and I can toss  out my relationship with my  friend Mr A with no qualms. I am begining to see  the hand writing on the wall that  my sister's best  friend may not take it kindly with us.  If she would not have my friend,  Mr A, let  nobody have him. 

what do you expect

That guy is not worth the stress.but u have to treat this issue with a matured mind.watch from a distance and let it all happen on its on.don't play any role just sit and watch until it all comes to the notice of Ur sisters best friend what kinda guy your Friend is,let her know that u cannot vouch for him.