It's gross and I try to say no but he won't leave me alone help me.?

Okay now I know that this is going to sound gross but I need help so I'm going to explain it anyways. So about a year ago we adopted a 12 year old boy. Well lets just say he has a lot of problems. he started asking me out a while ago and for about seven times I said yes because I got tired of him pestering me about it. Soon I got so tired of it that I told the police but it didn't go through because no-one believed me. Now I have a boyfriend and I love him very much. But my brother keeps asking me to have with him and make out with him and I tell him no but he keeps to make deals and all this crappy stuff. How do I make it clear to him that what happened then is the past and its never going to happen again.