My sisters girlfriend is cheating on her?

I saw my sisters girlfriend cheating on her with another girl and when I tried to tell my sister she didnt believe me because she thinks I am just trying to break them up. She thinks I’m trying to break them up because I was a bit upset when I found out she i gay and she thinks I am just trying to ruin everything for her. But I am not trying to break them up (well I hope my sister has enough common sense to break up with someone cheating on her) but she does not believe me. How can I get her to believe me?

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