I feel sort of bad for feeling this way, but I'm so friggin' annoyed by my brother's GF!?

No, you're not wrong for feeling this way! And I understand, you just like to lay around the house sometimes with no makeup on and lazy attire and all that, so do i. And I know people may think "well you care too much about your looks, dont worry about that, worry about how people think of you on the inside blah blah" but i actually do care about how people think of me, i don't go over the top but yeah i make an effort to look good and stuff. anyways....getting off topic there...yes. she should give you more privacy. instead of having her always come to the house, ask your brother if he could go to her house sometimes. or they could meet somewhere instead.