Girlfriends brother likes me

Today I spent the day with my girlfriend and out friends, her brother was there. I’m 18 and so is my girl, her brother is 23 but has some mental health issues that mean he has a much younger mindset. He pulled me aside while we were out and told me he has a crush on me and I broke to him that I’m with his sister, he told me that if it doesn’t work out he’s open to us. I was like

I know he has a lot of mental illness that cause him to maybe not understand that that’s not really an ok thing to say and not realize that the age gap is weird, but like I don’t know how to really handle this situation because my girlfriend loves her brother and I want to get along with her family. I hope he just drops it and if he doesn’t I will ask her to talk to him. (I already told her so she knows this happened)