I don't see anything wrong with him proposing to her the night before the cousin's wedding. It's not as tacky as proposing to someone during the wedding reception (a la "The Best Man").
I say mind your own business. But if you insist on saying something, you can make it tongue-in-cheek like "uh oh that's definitely going to steal the show from...." If there's a point to be grasped, he's probably smart enough to grasp it. If your brother-in-law is anything like my friends, family, and me we would tell you to mind your own damn business if you said it anything other than tongue-in-cheek.
ETA: And it really doesn't have to steal the show from the cousin. If the family lets that happen, that's because the family sucks (
). The couple who is getting engaged can make this as personal and private as they want to and respectfully request that the family not get all hooplahha about it until after the wedding.