Sleeping with my Brother's Best Friend

Is it ok to hook up with my brother's bestie? My brother is only two years older than I am and in the past he has messed around with my friends. He dated my best friend for a while and even moved in with her causing a rift in our friendship. But I became single a while ago and naturally I was drawn to his best firend whom I've always thought was cute. My brother doesn't approve, so we have been hooking up without him knowing. So far it's just sex since neither one of us wants to tell my brother what's going on...