Are my brother and my girlfriend flirting?

Yes, kick his ***, but not for the reason you think you should. You should kick his e because he's a phallus anyway. I say that because he's a moron who will never amount to nuthin' but the chicks, including your ditzed-brained, pie-eating, chronic infection of a girlfriend had the gall to suck his nutz while you were at school. I saw them doin' it, dude! I was peakin' in the window at the time, and man, your GF is HOT!