If your girlfriend is a hottie and her brother is just a couple of years younger then it would be a suprise if he
didn't want to fuck her. (And even more so if you all hang around together, he sees you and her 'making out' and he can even hear you and big sis making the beast with two backs
)
Undoubtedly he has been wanking off to her ever since he's been old enough to cum; the close presence of a slightly older female who is sexually alluring cannot be understimated regardless of them having almost identical genes.
The chances are that lil bro has used her panties to wank over and that for some years big sis has been writ large in his sexual fantasies, Even more so right now if he doesn't have a girlfriend of his own right now.
The best thing you can do - in the short term - is to let him listen to you and your girlfriend fucking in order to give him something to get off on. Other than that tease (mind-fuck?) can't your girlfriend set him with one of her friends? (Then you can perve over his woman :giggle
.
