Unsure on what to do.

So there's a lot of information so I'll bullet point a few to make it easier to understand 
1. I know a guy 7 years and he's also my brothers best mate
2. This guy fancied me for 4 of those years ( we've also made out) 
3. I move away but when he visits my brother we hang out and get along amazingly
4. He still flirts with me but idk if he still likes me or just wants to hook up
5. The same guy has started 'texting' my bestfriends little sister 
They've met up once and never kissed, and have known eachother for a few months 
6. She said she didn't like him and vice versa a week ago
So I texted this guy hoping to reconnect or hook up (going through a break up ATM) (ATM means 2 months ago ha) 
7. Then she decides out of nowhere that she likes him (even though they've never hung out)
.. By now I've started to like him .. Again
8. My bestfriend is telling me she'll find someone else for me ect ect 
But I've known this guy for years and have history with him and feel like he likes me more than her as he was flirting with me and told me he didn't like the other girl 
Confusing I know 
I don't know what to do.. Should I back off as it might be a rebound and it's my bestfriends lil sis 
Or will I go for it and talk to the girl about how I know him longer and all.. 
I don't want to be selfish