Hey there,
As I am a guy,I think I can give you the advice from a guy's view.Well I think you must stay with your boyfriend because you have known him from quite sometime whereas the other guy is new.He may not know you like your boyfriend.And another thing is that your best friend may not like you going out with her brother.She might think it's awkward.Also love needs sometime to become strong so I think the new guy is not perfect for you.
But if you really like the new guy,ask your BFF if it's ok if you go out with her brother.
I hope this helped
All the best
Liadeon