The House of Yes
Parker Posey is a mentally ill young lady, and when her beloved brother brings home his new fiancee to meet the family, she is not amused. A very, very dark comedy. And Tori Spelling is in it too! Originally a play, so the pacing is very long scenes with fantastic back-and-forth dialogue. One of my very favorite movies of all time.
Note: don't read too much on the IMDB page, there are spoilers in the Quotes section. Trust me, just watch the movie.