The Vicious Kind (2009)

Genre: Comedy | Drama
Tagline: How would you protect someone from their first love?
Starring: Adam Scott, Brittany Snow, Alex Frost and J.K. Simmons.

Plot: A man tries to warn his brother away from the new girlfriend he brings home during Thanksgiving, but ends up becoming infatuated with her in the process.

http://www.imdb.com/title/tt1183921/
http://en.wikipedia.org/wiki/The_Vicious_Kind