I'm looking for a romantic comedy movie. ?!

I forgot the name of the movie and how played in the movie!. I remember there was a guy who had a one night stand with a girl that turned out to be his fiancee sister!. Please help me find this movie!. I do remember one part when the sister came over the parents house and he looked himself in the bathroom!. The sister had a crazy ex-boyfriend police office!. Very funny movie!.!.!.Please help!.!.!.Www@Enter-QA@Com

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