Originally Posted by
feelsohelpless 
Now the question is how heartless should I be? Do I cease all contact once and for all or do I stay in contact with him as a friend. We have history, we still love each other, it's hard to imagine no contact whatsoever.
My experience is if one party doesn't want to break up, then it's best to go non-contact except for when necessary to talk. It's much harder for everyone otherwise, and strings out the pain.
Contact can be possible if both parties are agreed, which isn't your case.
It might be wise to change the locks at your place in case the sister gets hold of his keys.