I think my brother’s relationship is weird because the age gap

He’s been dating this 18 year old and he’s turning 26 next month. He says they met on a dating site but the girl is still in high school, she’s a senior. I didn’t really care about it and just found her to be cringe because the way she infantilizes herself and the way she talks to him. I’m closer in age to her and yet I still see her has a dumb teen. I’ve been side eyeing my brother because it just doesn’t look right and I don’t see how he can take her seriously.