boyfriends brothers girlfriend

I have to vent about this somewhere ... I can't help it, I hate my boyfriend's brother's girlfriend. It's like his attention span changes when she's in the room and he's talking to her the most. Last night, we were all sitting at a table and she kept giggling like a retard about nothing and she made him laugh really hard... I've never made him laugh like that and it hurt me when that happened. Idk why. I got jealous. Like really jealous. I'm very insecure when other girls come around because I always want my boyfriends attention on me. Is that bad? Is it bad I hate someone who hasn't done anything to me? Please someone give me advice.