OP's wife: As stupid as it sounds now, I was trying to plan a big surprise for his birthday and he has lots of siblings so I thought it would be nice to get them all together. I didn't have contact details for a lot of them and his mother is the one that had organised previous birthday parties for his siblings so I went that route thinking she could help me put something together. I got in touch with her months in advance and she told me it wasn't enough notice (I am not sure what enough notice would have been?). The travel wasn't a money or time issue and while it is a couple hours by train she and the rest of my husband's family frequently travel.
As far as bringing out a bad reaction in me, I completely agree and I feel like she would have wanted that. On the wedding day I was focused on getting married, nervous about the ceremony, excited about the whole thing, and concentrating on being with my husband so she was a pretty small part of the actual day for me. I didn't particularly want her there given her past behaviour but I was willing to have her there because at the time it was something that was important to my husband and my love for him beats my distrust in her. So, she didn't get a reaction from me and I am glad of it. To be honest, it probably bothers her than I didn't get upset on the day, and I am fine with that bothering her.
My husband was really trusting of her and I think she took advantage of that. I know he didn't mean to allow her there and didn't think she would behave like that.
OP: The birthday party was a complete surprise and my wife organised it so I was unaware. The excuse the rest of my family (father and siblings) gave for not coming was it was my nephew's first birthday (them and I share a birthday). At the time the excuse given to my wife was just 'we're busy - too short notice' and only after the event was it mentioned that my family would rather attend nephew's birthday. Not going to lie, that hurt a bit.
You are right, my wife was right about warning me about her (potential) behaviour at the wedding. Even my father warned me previous to the wedding. As he put it (tactfully to my mother as he was married to her at the time) 'If you feel that your wedding would be a happier event without us then that is absolutely fine'. I wish I had listened to what he was actually saying. It seems so obvious to me now, but I genuinely thought that my mum loved me and supported me. Lesson learned, but sadly it was too late and I regret it so much.