NotSoHappyBeet I have spent much of today looking through the messages, once I realised my mistake (above!). Again apologies for not reading all the messages, I have now tried to read all the ones from you.
You said your mum had said "Of course she loves my Dad, she just isn't IN love with him. (I told her I'd heard that line in most cheesey movies but she still insists it's true)" I think (IMHO) you should give your mum the benefit of the doubt in terms of the love she does, or doesn't feel, for your dad.
I mean you just don't know. Of course having an affair is a very cruel and awful thing to do to anyone. But I am guessing (and I have not had an affair so I don't know) but I am guessing someone who is having an affair just tries to put their spouse out of their mind and is consumed by the affair. That sounds awful, I know, but it doesn't mean that they do not love their spouse.
It sounds very sad she is not able to be truthful with you about the OM's wife and her extra marital affair, but she is presumably trying some damage limitation, although this is stupid it is, to me, understandable.
I can totally understand about her being mortified about you telling your other half but I also understand that you needed support.
I do think it is very positive when you say "She asked me then what SHE should do."
I said if she cared anything about my father at all then she should call everything off with OM (if that's even up to her anymore, still no contact apparently)
OR if she really didn't want to be with my father then she should tell him that and go from there, not about the cheating but how she feels."
I felt very sad by your advice to your mum, I can see you think this is for the best and that he is being "treated like a fool" but I don't think he is and if your mum is his main/only carer I am not sure what would happen to your dad if she did leave him. Indeed I personally think leaving him in these circumstances would be worse than staying and continuing the affair. not that affairs are good, but it is the lesser or two evils in my mind. And I am not simply supporting her because she is the woman, I would feel the same if your mum was ill and your dad had had the affair.
You said Finished up by telling her I'd lost most of my respect for her over the last 24 hours, that to honest I'm not sure where our relationship goes from here but I want some space away from her for a while. That is of course your prerogative but please do try and put yourself in your mum's shoes for one minute... she has a very ill husband, I have no idea what the reality of their relationship is but for whatever reason she has had an affair and now her daughter has found out and the whole thing is mortifying for her.
Of course you are drained by all this, please look after yourself.
Please, please do try and see your mum's side of this a little. You said She said I should remember she loves me. I am sure she does. Please do not allow this to destroy you and your mum's relationship, and I would personally let your mum work this one out, if you can bear to. I am saying this because I think this will be best for all of all - for your mum, your dad, and you.
We may love our parents, we may expect to look up to them and when they let us down, badly, it must be an appalling shock. But there are much, much worse things that parents do to their kids and for your own sanity and health I would try and see outside of this and see the bigger picture if you are able to.
There is lots of advice on here and maybe lot of people saying things that will help you and make you feel better in the short term (justified in your actions and in your anger) but in the long run this is not an insurmountable problem, IMHO. But you do need to step back and allow your mum to deal with this. I think if your dad does find out, and finds out you know, then I think you can say in all honesty you did what you though was for the best in a very difficult situation. If you force a confrontation, you may not be able to say that.
Castrovalva (up thread) had some very wise words.