Worried About Brothers Ex Girlfriend At His Wedding

My brother is getting married and I have just found out his ex girlfriend from 20 years ago will be there. Whilst my brother was with her he suffered clinical depression (not to do with her) and he had to go into hospital at that time. My mum, who has also suffered with depression, tried to help him but the ex girlfriend asked her to stay away which was extremely difficult for my mum, so much so she still talks about it today. 20 years on my brother has been with his fiancé for 10 years and they have two children together. My brother, for the most part I believe, is well now and has been off his meds for depression for a long time. I’m surprised he and his fiancé would want to have his ex at their wedding as the break up was quite bad and she couldn’t deal with his depression in the end so my mum did take him in and looked after him in the end but that’s up to them. I’m worried about how this will make my mum feel as she is so happy and excited about the wedding but this will be quite a shock to see his ex again. I’ve never know my mum to dislike anyone (ever) apart from this person. I don’t think I can talk to my brother or his fiancé about it as they wouldn’t understand and they can invite who they like but how can I help my mum with this? It’s an important day for her and I believe this may ruin it. I also feel a dislike for this person so I’m trying to manage my own feelings whilst keeping it in my head it’s my brothers day and not mine or my mum’s.