I think thats fair, his self-confidence is low because everyone rubs in his face how inferior to his brother he is so having a girl be this crazy about him is good. The fact that his brother is in love with her is an added bonus to make the situation feel better.
she's in high school, he's a working adult, assuming she's in her first year she is 16 and he probably is around 22-24. Not a too big age gap and as they know each other for years then theres nothing weird about it tbh.
Dahlia is no joke heh? I mean we knew she was stronger than the others as she was able to effortlessly handle berserker Yakumo but now we know that she's kind of the Satoru Gojo of that universe maybe?