OMG! What do I do my bfs brother has a crush on me and won't leave me alone?

My boyfriends younger brother Sam, who is a year older than me has been bugging me for the last 2 or so weeks trying to get me to flirt and talk dirty with him but I keep turning him down. But I guess he likes it and thinks I'm playing 'hard to get' like i try being a ***** and it doesn't even deter him he thinks its a little act I'm putting on but I do like him. My bf is aware that his brother was flirting with me but told me not to mind him he's just having fun and not to take him serious but yesterday Sam told me he actually has a crush on me. I told my bf and he seemed kinda pissed off. I don't know what to do I know since Sam's been getting more insistent with me they've been arguing more but my bf seemed genuinely mad last night I don't want to get in between two brothers and ruin a relationship. what should I do?