<human>: So apparently my older sister (25) and my mum already know and wanted us to know too (dad doesnt know how older sister found out). He says he goes and visits her sometimes, but he isn't planning on leaving us or anything. He has also said to ask him if we have any questions about it. I didn't even know he had cheated on my mum.

My mum and my dad went through a bit of a rocky phase where he cheated on her about 17 years ago now, all I remember was lots of arguing and dad went to stay in a hotel for a while. I have no clue if thats related or not and I dont even know if I want to know. He used to travel a fair bit for work, so had plenty of times something could have happened. 

I also have a vague feeling my older sister might have already told me, but if she had I had packed it away in a box and this time was news to me and so I dont know if this is my memory playing tricks on me.

Has anyone else ever been through something like this? I dont know whether or not I want to know who the woman is, how they met, whether he knew her from when we lived there, whether it was something that had been going on for a long time or whether it was a fling, why the hell he didnt use protection while he was putting my family at risk. Hell, even why he did it. 

Does anyone have any tips for mentally processing it? I can kind of already feel myself putting this away in a box and pretending it isn't a thing and I just don't think that does anyone any favours. I actually care much more about how he could cheat on my mum again (and we had all gone to boarding school at this point so she would have felt so alone) than the fact that a child came of it. 

I am probably going to write him an email if I do want to open a dialogue about it because I am not very good with words.

Can you provide a brief rundown of the text in within 3 sentences?
<bot>: My dad has a daughter from when he cheated on my mum, I don't know if I want to know anything about it or how to process it.