When I was dating my now-husband, his sister made some comment to me that any normal person would have taken as an imminent proposal. I kept my mouth shut. Good thing, because she was just being a, well, you know - the B word. It didn't happen when she said it would. It was 3 months later.
Are you 100% certain that he is going to do it? Is his cousin being completely honest with you?
Proposing to your future wife is something that (hopefully) only happens once in a lifetime and is usually a surprise. Do you really want to steal that from him?
It's hard to do, but lay low.