confront your mother, don't tell your dad. give her a chance to turn things around before it breaks the family. If your mother still cheats on ur dad after you confront her, then tell your dad. I hope this works, heh heh... 

Why would you seriously want to act like you don't know? That is one of the worst things you can do. In a family, the three most important things are love, trust and communication. Obviously you don't want to hurt your dad, but you are making things even worse by acting like nothing is wrong. You are sick. I agree with #27.

Tell you mum to tell your dad, or you will. Its simply not fair on your father to keep quiet.

No, you don't. Your dad will hurt more if he finds out you knew and betrayed him by not telling him. Tell you mom she had 1 week to tell him or you will. Your father has done nothing wrong.

Seriously everyone saying she shouldn't have been snooping shut up. Relationships I figured are meant to be based on trust and clearly your dad can't trust your mum (or mom as you're American). She shouldn't be keeping secrets from him. Give her an ultimatum then tell him yourself and show him the phone if she won't.

The same thing happened to my sister in-law last September. She got snoopy and found cell phone papers and a love note from the other guy in her mom's drawer. While I HATE people who snoop, I'm not sure what I would do in that situation. Because of all that, this family is turned upside down and my children rarely see their grandmother.

Your dad may not be your real dad. If your mom is a sleazebag now, she likely was before as well.

Wow that's an awful position for you to be in! I feel so bad for you. But you really should confront your mom. Is there a possibility you misread the texts?

You do NOT have to pretend not to know.

give dad the phone.