Whilst you can sympathise with your mum about the difficult divorce, I think you do also need to respect the fact your father is now married to the OW, particularly if you want a good ongoing relationship with him
It is very kind of you to consider your mum's feelings but I disagree that "it's not fair on her" to expect her to be an adult and put your feelings first for one day. Is she likely to kick off if you do invite the new wife? Will your mum have other people around to support her? Obviously dont seat them together, but otherwise id hope they could just be civil and/or just avoid each other
It does however depend how long ago this all happened? If it was within last couple of years, it may be more raw all round i know, but if it happened years ago... your mum is fully entitled to hate him and never forgive him for his behaviour but equally needs to let work on letting go of the heartache for her own sake xxx
Also, we say this to kids all the time, but please remember even as an adult, you are not responsible for your parents past behaviour and/or feelings around their divorce or relationship.
If you know he cheated "multiple times", presumably your mum also knew this and chose to try again multiple times, voluntarily, not saying that it makes it ok at all! But... it's really TMI for any child to know so much about their parents breakup as even with obvious wrongs such as cheating, their relationship was theirs and you won't know every thing about that dynamic and how it felt for each of your parents throughout etc.
Good luck with your wedding x