<human>: I found out that my dad was at a certain place with another woman out of coincidence. He didn't see me at anytime.
I was with my GF when I found out, I then managed to get inside the place and found out the shower was wet and towels where used. 

I didn't know what to do, so I decided to meet the next day with my dad and tell him what I saw. At first he denied it, but after I explained the details he accepted it. 

He told me that he is not happy with my mother, that my mother gives all her attention to me and my brothers that they barely have sex and what shocked me the most, he has been cheating on my mother with multiple different women even before they got married.

He says he is not in love with some other women nor anything, that its just physical. He also said that pretty much the only reason he is still with my mother is because of my little brother.

My dad asked me if anyone else knew, and I told him that my GF, he also said that he doesn't want my mother to know because its going to hurt her, that's when I told him that he is the one who is hurting her, not the person telling her.

My GF kept telling me to do something about it and tell my mother, that if I don't its because I'm ok with my father's behavior (Which I'm not) but I don't know if I can tell her. I don't want to be the one who breaks the whole family apart, makes my mother go in pain and change the view my brothers have of my father. I know they are never going to see him the same way.

What do I do? I've been carrying this for months and I don't know what to do anymore.

Can you condense the text into a few key points?
<bot>: Dad has been cheating on mom for all their relationship. I've already talked to my dad about it, what to do?