issue thats hurting me inside

Girl I need your opinion on something. Myrother has a girlfriend, and she went to her dad's for the week, to get away for a while,and he has a girl over, at our house. She stayed the night and they slept together, and he had his arms around her, and she flirts back.....do I tell my best friend (his girlfriend) or do I just let it happen ?? Its killing me to see him flirt with another girl when he has been dating someone for over a year now. I want to tell her so bad, but I don't want to hurt her because sheis my best friend. help please

I would say as much as it will hurt her, she needs to know. I know I would want to know.

That's the hardest part I love her so much? She is like a sister to me, and the last thing I want to do is hurt her :'(

I think she needs to know. It may hurt her, but I would rather find out sooner rather than later.

(had to reply to this)
I think it is gonna hurt her more if you dont tell her and she finds out and asks you....i would want to know if i were her and its not like its your fault he done that....i would tell tell her...

Thanks girls. I'm going to tell her face to face. She won't be back til Sunday though if things work out. She got bit by a copperhead not a good day for her