Why do some guys make sexual jokes about their brother’s girlfriend?

I have a long-distance boyfriend (he's 21), and he has an older brother who is 27. l've met his brother a few times, he's always been nice to me, and my boyfriend told me that he just makes dumb jokes sometimes. The brother says things like "Ah, he's gonna get pussy soon" or "What are you gonna do in the room with her?" to my boyfriend. What's going on in their heads when they say this? My bf said everyone in the family likes me but I wonder maybe he actually hates me 🤣