Hmm. She sounds like my cousin. My cousin spent our childhood belittling me, and treating me like crap.
The strange thing is tho... looking back at our childhood together.... I realized that my cousin was never
INTENTIONALLY malicious. I think she just didn't realize what kind of hurtful moronic things were coming out of her mouth.
It's very possible that this girl is just... incredibly dim-witted and thick, and doesn't realize the things she says to you are horrible and hurtful. I can't see why else she would invite you over for a social occasion... she MUST think you're friends.
That being said... if my mother hadn't put the two of us together when we were kids, I probably wouldn't've willingly socialized with my cousin, as she was rude, and made me feel bad. So I don't know why you would socialize with this girl outside of work out of a sense of obligation. If she makes you feel bad about yourself with her b!tchtastical comments... why would you put up with that?
Whatever you decide... we're behind ya.