<human>: Summarize the given text in a few sentences.

Sorry for the semi-wall of text, to start off my gf likes to party and go out with her friends occasionally, which I don't mind at all. We have been dating for 2 1/2 years and share everything with each other. Her and my sister along with a bunch of their friends went out the other night to a local bar. My sister and her friends gang up on my gf and start accusing her of cheating. I pick her up, she claims that its all false rumors. I speak to my sister the next morning and she claims it has been going on for months but she has no evidence, just people coming up to her and telling her that my gf cheated on your brother x amount of times. I don't know who to believe. There is no evidence of the cheating, just all talk. My girlfriend denies it endlessly. What should I do?
<bot>: sister told me my gf has been cheating on me for sometime but has no evidence and my gf has been denying the rumors.