My mother has been having an affair for 15+ years and I don't know what to do?

Almost exactly two years ago I found out my mother was having and affair, I picked up the house phone the same time she did, but she didn't realise I was on the line, I was going to hang up until the man she was talking to started asking really strange questions like 'where are you, what does your bedroom look like what colour are your curtains' etc and then went on asking the weird questions that got more personal until I started to become really suspicious, in the heat of the moment i went down to my dad and told him that my mum was talking to someone funny on the phone, he didn't seem particularly bothered but asked me to try and find out who it was, so i picked up the phone again and they were talking about meeting up and my mother saying how she would try and get away. I put the phone down and went to the outside of he bedroom door, she heard me and pretended to be talking her elderly friend. But at some point during all this she realised i'd figured it out, and called me to her room. She told me it had been going on for a long time, she told me that part of the reason is that my father is impotent and that was part of the reason why, and she told me to never ever tell him..or anyone. I asked her to stop, she said she'll have to now, after wards I went down to my dad and told him that i had been wrong about that person on the phone, i'd got it wrong. Every thing settled down after that I didn't want to ever mention it, because it was too awkward and I tried to take her word for it she'd ended it. But then recently I was going to a concert, and i wanted to record videos with good sound Quality because neither my phone or camera have good sound quality for things like concerts, only my old phone which is now my mum's phone, so i took it with me, she seemed okay with that, but when i went to send a text from it to my sister i saw a name in her inbox that i didn't recognise, it was a girl's name, Louise, and i couldn't think of a louise she knew, I opened it and found these explicit texts, they almost made me physically sick, and things like 'great seeing you last night', and 'come and see me when i'm alone at work' and some other sick sexual innuendos, but I also now know that they see a lot more of each other than i ever thought they did, and whats more my mother goes out 3-4 evenings a week to play card games (bridge) and what disturbs me is this guy, i've discovered from the texts, plays bridge with her, so when we think she's out at bridge she's really with this man. I've tried to act nomally around her again but its really getting to me, and now my sister is the same age i was when I found out (15) and i'm wondering whether she deserves to know or whether it would be crueller to my father if he's the only one in the family with out a clue, what should i do?