I'm in love with my best friend

I've been friends with this guy my whole life, we've had crushes on eachother on and off, but he has always been one of my best friends because our families are also close. Lately we've been hanging out a lot and talking everyday, it seems like he's flirting. I don't know how to act around him or how to move things forward in whatever the hellwe're doing, or if I'm getting the wrong signals! I don't want to ruin my friendship with him or my friend who dated him briefly in the past(she broke up with him and claims she never liked him but gets mad when I talk about it, even though she's gay and in love with her gf now) PLEASE ADVICE 🙏🏻