I saw my sister's boyfriend cheating on her

i think you should talk to him first.

You have to tell her. You'd want her to do the same for you.

It's family, tell her. She'll likely hate you for it but you should tell her anyway because not telling her is a far bigger betrayal.

If there's a bad vibe I think you should break it up.

if you know for a fact he cheated, tell her. don't bother telling him! he will just denie it.

Well, would you rather see your sister hurt while catching her boyfriend messing around with another girl, or would you tell her ahead of time so she can go figure out what she wants to do with him?

Even if she's a hard-headed girl you should tell her what you saw. looking out for family always comes first you know..

Tell her the truth its for her own good

I THANK You SHOULD TELL HER ! LET HER NO WHATSUP BECAUSE EVEN DOE You FiLL AS IF SHE WOULD LISTEN SHE WOULD STILL NO

I think you should tell her, even though she might not believe you and will definitley be hurt.
She deserves to know.

i wouldn't even hesitate to tell her, it will hurt her even more to hear it from someone else. why would you want her to be with someone who treats her like that. kick him to the curb and don't feel bad about telling her, you are doing her a favor by not wasting her time being with a jerk.
he will regret it and she will move on.
lifes little lessons
=)
please tell her. its for the best.

You should tell your sis she is your blood and even if she is hurt and angry at first she will never hold it against you. In the long run she will always love you more for looking out for her. My sister always looked out for me and I am so thankful to her for that we are best friends. It always sucks to know that your sister knew he was cheating and never said a thing.