Cheating on my little sister!!

Make sure first that your sister is unaware in what her boyfriend do because we are not sure if the girl that his being with is his sister or school friend. If you prove that your sister don't know what his boyfriend doing then tell it to her. There is nothing wrong to tell to your sister what you see if you think it will help her. Cheating is a very bad thing in a relationship..

well, it wont affect your sister anyhow, atmost she will tell you that you are jealous of my Friend, Another thing is, if he was watching movie with some other girl, that doesnt mean that he has affair with other girl or sumthing. you can inform your sister, in non-complaining way, just talk with her casually and say, "Oh i remembered, i saw your BF in theater yesterday, she was with a girl."

Would you want her to tell you if the roles were reversed here? If the answer is yes then I would tell her but I would be very careful how you choose to do it. Maybe you can casually say that you saw him at the movies with a friend and when she asks who you can say you weren't sure of HER name but you have seen her before! lol. I know with my own sister, when we were younger she probably would assume I was trying to give her a hard time rather than help her in a situation like this so I would choose my approach carefully!

You should tell her for sure! Just put yourself in her shoes... if she had found your lover with someone else, wouldn't you want to know? What she does with the information is up to her. Hopefully, she won't "shoot the messenger". Just remember that if she's upset, it's not because of you or what you did, but the information you're relaying. Just be there for her to support her in whatever decision she makes.