When we married, my mum pulled the 'if he/she comes to your wedding, I won't ' card. Stbdh had a sore with her, along the lines of ' Banana would love to have her whole family at her wedding. She understands that you and X do not get on, but there is no reason why you would have to speak to X if you choose not to. If you really feel you can't attend, Banana will be sad, but she will try to understand why her mother puts a petty argument ahead of her daughter's wedding day ' . My mum came to our wedding and she and X actually built some bridges that day. So, I'd call her bluff.
Book her a place. Tell her it's there and you hope she will come. Then leave her and hopefully she will be able to turn up without losing too much face. If she comes make sure you make a fuss of her, she may find it quite difficult if she feels pushed aside by your relationship with your aunt. Hope it all works itself out.
She knows her place will be there at the top table, where she should be. It's just so upsetting. My father passed away two years ago and it'll be hard enough him not being there. My brother will be walking me down the aisle and it'd be such a proud moment for her. I just can't understand why she is being like this. No one can get through to her.
Your mum needs to grow up. Unless your Aunt has done something unspeakably bad to her, then this isn’t your problem - it’s hers. I assume that whatever they’ve fallen out about didn’t concern you, or cause you upset (ie. you have no personal reason to dislike your aunt?)
Leave it to your mum, but perhaps make it clear that this will irreparably damage your relationship too. Because it will - she’s putting a row before her actual daughter.
Agree with everyone else. You don't need to do anything. She needs to put her dispute aside and realise this is your day, and she will regret it for the rest of her life if she doesn't come. However, don't let her emotionally blackmail you, it is her that needs to realise this day is more important than whatever her dispute is about.