secrets and trust

Wow you are awfully young to be carrying around such a heavy burden. The thing about coming forth with the info to the person being cheated on is YOU are usually the one who gets the brunt of the anger, not him. You’re viewed as the troublemaker, home wrecker, or a liar, accused of trying to be hurtful or spiteful and sometimes end up shunned.
If this was a friend I would say absolutely tell her, but this is your mom and the fallout may affect your whole family. It’s not just the one person being affected, it’s possibly changing several lives and going to hurt several people…. possibly irreverseably. It may destroy your relationship with your mom, your dad, and your sisters. Do you have close and decent relationships with your mom and sisters? If you do have a decent relationship with one of your older sisters maybe talk to her about it first or if you are super close with your mom, I think you should tell her.
I think she should know. If it were me and my daughter knew and had proof as well, I would want to know. But coming forth with this you need to be aware and prepared for the fallout to be bad, it’s going to hurt. It could break up your family so don’t disillusion yourself and think everything will be ok cause it may not be. I think your mom has a right to know and then she can choose what she wants to do about it, confront him or not, stay with him or file for divorce. I just hope if you do she sees it as you caring about her and wanting the best for her and she realizes the burden you’ve been carrying and understands. I just know a secret like this would eat me up inside and this is something that can affect YOU in a lot of negative ways both mentally and physically and that’s not good for you.
There is another option and I don’t know if you will feel comfortable with it but you could also go to your dad and tell him that you know, that you have proof and say he needs to tell your mom or you will, give him the chance to tell her himself and take the responsibility off your shoulders. I don’t know how close you are to your dad so be aware that will probably affect your relationship with him, but why should you have to be the bad guy in this? It’s his mistake, not yours. He made the decision to cheat not you. That way you are not so much in the line of fire so to speak and the blame and anger will go where it should if he does the right thing.
It’s a tough situation to be in and there is no good answer or easy one, someone ends up hurt no matter what…. but I really do worry about you keeping it to yourself and what it’s going to do to you in the long run.