You gotta remember that he's the one who provides her with the romance and a lot of the excitement in her life. She's probably put you and your siblings' happiness before hers for so long, but she deserves to be happy too so you can't blame her for not wanting to mess a good thing up. (This is as long as he's not a jerk and it hasn't become child neglect.)
If you tried to respect him, I bet she'd appreciate it and in turn be more receptive to what you have to say.
