boyfriends brothers girlfriend

So I have only been dating my boyfriend for about a half year (6 months), but I've known them for seven years (me and my boyfriend were best friends before). My boyfriend has an older brother and he recently broke up with this girl he was dating for three years and started dating this new girl just two days after that. My boyfriend and I are only seniors in high school and his brother graduated high school two years ago so my boyfriends brother still lives with his dad. His girlfriend, who is my age, sleeps at my boyfriends dads house every weekend and my parents don't let me have sleep overs every weekend so I can't sleep there a ton. My boyfriend's brother's girlfriend likes to talk to me like I'm new to the family and like I don't know anything about them, when I know for a fact that she doesn't know much. She also tries to tell me things about my own boyfriend as if I didn't already know them. She also only sees their dad and never their mom or 7 year old brother, which I see both parents all the time, they're divorced so they don't live together. She doesn't even try to see his mom. She really annoys me and I want my boyfriends brother to break up with her so bad, but like he loves her and so does their dad, but I really just wish she was gone. I really don't like her. Help me I feel like I sound crazy. 
**ps I'm sorry if that was too confusing for you guys