It’s weird because of the age gap, sure. He met and knew her when she was a child and he was a fully-functioning, presumably sexually active adult. Not saying he’s done anything wrong exactly, but it is weird.
I think i’d struggle with a mate having a relationship with my sister if i had one, tbh, irrespecrtive of age difference. At some point in your life you’re going to have to weigh your sister’s boyfriend in, so it helps if he’s not your best pal.