I had sex with my step dad's brother and I feel disgusted, now he is threatening to post it on facebook?

Last night I had sex with my mum's new husbands brother. He is only 3 years older than me and a total bum. He does very little with his life but get high and surf. I do not understand why I did this. I am a university student and I should have been smarter than this. The problem is he is now saying that he is going to post it on his facebook page so my mum can find out via his brother. What the hell am I going to do.