Jealousy

I don't know why, but I always get jealous when my boyfriend is with his family. I don't know why I feel son jealous but everytime he is with his mom (who spoils him like no other, and my mom doesn't) I just get so ANGRY. I don't understand how, but I always feel like he will always lose interest and talk bad about me. His sister who is spoiled will also cling on to him and try to break us up somehow. I feel like they're trying to pull him away from me and I'm trying my hardest to be the best girlfriend I can be. Any advice on how I can get over this?