Well, no, he wouldn't be able to tell at first, but if they interacted he'd probably pick it up pretty quick, because Kiyoshi would know of Kariya (his eldest brother is named 'Kariya', and I'm sure they'd know why) and have a rather-idealised view of him.
As for the "arranged marriage" thing, I think that notion would be pretty quickly dealt with, and even if not I would expect Kariya to still want Sakura's kids to be safe, especially given that the kid clearly adores his mother.