He says "just ignore her, she'll grow out of it"....and I entirely disagree. He doesn't try to argue about it because he understands how I feel but he just sees it as his little sister being a pain.
Well, I guess the bottom line is, as much as you hate it, she is not your responsibility...
But if she sets foot in your house, she is under your rules. And make sure she knows that.
If she doesn't appreciate it, kick her to the curb.