Help on stupid stupid girls?

So, i have a BIG problem with this one girl.. lets call her lisa. Lisa, wanted to meet me SO bad in the beginning of this year, but after that we had problems. Basiclly, she 3d everything up. I honestly did nothing. I can't stand her, but she just seems to show up everywhere.And it drives me insane. It really shouldnt because since i don't like her, you woudl think i would wnt to forget about her or atleast try to? But i cant. Well, recently she started dating my boyfriends brother. I know, that would happen to me, right? Well it sucks. But i always complain to my boyfriend abotu hwo much i CANT stand her. I actually want to hurt her most the time. But he just doesnt care? I guess causee i just obsess over it a lot.. or because its now his brothers girlfriend so maybe it seems like he has to respect her? I dont know. Oh and she used to like my now current boyfriend (obsessed with him actually) before we started dating, about 11 months ago. I just need help to understand what i should do or why my boyfriend just seems like he takes their side? I do need help to try and deal with this ***** though.