How to disapprove of someone else's proposal?

If she is happy that should be enough for you. You don't have to agree with it but you will need to let her know this. He may be an a$$ in your eyes but he may be next to God in hers. But as your sis she should respect your position and take it all in. Even if she goes through with it at least she knows you aren't hiding your true feelings. And over time if she stays happy, you may change your opinion of the guy.