parsleythelion, I can imagine how you feel... my parents divorced because my dad had an affair with another woman.
However difficult it maybe, you've got to realise that it's their relationship, and what they do or do not do is up to them. Personally I would say that you should not tell you mum, especially if she is having other problems at the moment.
However, if you feel that you can, then I would confront your dad - it might move things on and help them get resolved - whichever way - sooner rather than later. This might make him see sense, and pack this other woman in, and it'll also be good that you can tell him exactly what you want to say to him, rather than keeping things bottled up inside, which isn't a good idea. Being 'caught' might be good for your did and make him realise what he's doing. And it'd be better than you blurting something out when you're mad at him - and if you choose when and where to do it, it'll not be (hopefully) when your mum or sister are around.
Finally, do you have a best friend or someone close you can talk to... that always helps me and makes me feel better - as they say a problem shared is a problem halved. Also, if your grandmother (dad's mum) is still around and you get on well with her, and can talk to her... maybe tell to her... my nan was great when my dad had his affair, she told him off like he was a little boy again. Whenever my dad did something wrong, I'd tell my nan and she'd sort him out. She was great!
Feel free to PM me. I'm sorry your having to go through this. I hope it all works out.