My mum’s cheating on my dad and I’m the only one who knows, help please?

I’m 16 and I live at home with my mum, dad and my 12 year old sister. I’ve suspected for a while as one of the contacts on her phone were just a group of letters rather than a name. Recently her phone has been pinging away and every time she reads the message she smiles and laughs. If I ask her about it, she would shrug it off.

A while ago when she was in the shower, I heard her phone go off so I decided to have a look just for clearance of mind and check that I was just paranoid. There were hundreds of messages dating all the way back to 2013. After a little bit of snooping, I discovered it was her old boss, who also has a daughter

My first option is to do nothing. I’ll be moving to university in a year anyways and I don’t want to leave for my sister to face all the consequences. I’m just a child and I shouldn’t be meddling in their relationship. However I feel guilty and I’m already struggling to pretend that everything is normal in front of my mum.

My second option is to tell my sister. we tell each other everything and if the roles were reversed, she would have come running to me. I would feel more comfortable knowing there’s someone else in the house who knows and she could maybe have an idea on how to approach this situation. However, she’s only 12 and I don’t want to upset her and drag her into this mess.

Do I confront my mum. This would show to her that I know and maybe persuade her to tell my dad. Realistically though, she will deny it and possibly blackmail me into not telling my dad. Our relationship will be ruined and things will never be the same.

I could tell my dad. I feel he deserves to know the truth and if divorce is the solution then surely that's better than my mum cheating on him? I just don't believe I have the mental strength to watch my dad break down in front of me, and I have absolutely no idea on how to break the news. I will immediately cause a tear in the family and I don't want to be held responsible for that.
My mum’s cheating on my dad and I’m the only one who knows, help please?
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