@Sammybunny40 I can think of two possibilities—1. He's trying to disqualify his brother via the girl, or 2., He's really a tsundere towards his brother and wants to see him happy. He knows that his brother only liked her (the prince's weakness), but he refuses to admit it, so to overcome that stubbornness, he's providing some
help.
I think that it is unclear which it is because the brother could be doing both, only one, or something else. His true motives are unclear. There are a bunch of
potential motives, but none specified as of yet (ambiguous motives). :3