Help! Catching feelings for my bf's best friend

So my bf and I have been together for almost 2 years. The relationship is great and we are super in love. His best friend is very very attractive, and from the beginning there was some chemistry between us. We got along great and my bf became jealous, however nothing happened even though it could have, I always respected my bf. Lately our relationship (me and the best friend) has been much more distant and I'm fine with that, it keeps my bf from feeling threatened, however, his best friend is now super close with his sister and every time the 4 of us hang out, I find myself getting angry and jealous of his sister because of how close they are, and I get bitchy with everyone in the group and my bf is starting to notice and get upset, he doesn't know why though. How can I get this guy out of my head and avoid conflicts in my relationship? I see him a lot because my boyfriend and him hang out all the time, and right now we're not talking a lot or anything but I can't help feeling insanely jealous when I see him being close with another girl.