dad cheating

Idk if i should be sharing this, but I am going to. I am really stressed at the moment, I went through my dads phone two days ago because I thought something was up. He recently got Snapchat and I thought it was kinda weird so I went on it. (I’m thirteen, nearly fourteen) and I found texts between this random lady I’ve never seen before and my dad. It was reall sexual and I found a bunch of nudes. So my dad cheated on my mum. I’m devastated, I really don’t want my mum to know because my whole emily will be torn apart. Last night I was talking to a girl on here who has been really supportive of me (she’s 17) and she told me if I was in my mums shoes I’d want to know. i feel sick and last night I told my dad he should tell her and he just went through a bunch of reasons not to. I can hold it in, but I don’t feel it’s right, and because I’m so young I feel like a have a big responsibility I shouldn’t have. Can you guys please please be supportive and give me advice. I am in a really difficult situation right now and I don’t think I can hold on.

Edit- *4 hours after I posted

My dad and I have decided to tell her. I feel sick to my stomach and I already know what’s going to happen. My mums been hurt like this before d she’s working late tonight, so after all the other kids have gone to bed (I have two baby sisters and one 12 years old brother) and when she comes home he will tell her. I won’t be there but I’m going to be awake to see if anything comes up