An old friend of mine is single, I want to contact him, but I have a problem..Help me?

He recently went to prison. I recently found out through mutual friends that he's a single dude. He has a year left in the penitentiary. He has no idea I know that he is prison and I do want to contact him. This old friend of mine use to live with me for a month and a half before he moved out of state. I haven't seen him in about 2 years and I miss him dearly. The thing is when he use to live with me, him and my sister had a one night stand and tried to be together. I didn't let them. Would that be wrong to try to get into a relationship with someone my sister was with? I didn't let them be together because he was at the time my best friend who I had deep feelings for and this was 5 years ago that it happened.