I would prefer for my ex to not be at my niece's wedding in all honesty.
What is the worst that can happen if your ex attends the wedding?
Yes, she hates my guts and will ruin it.
Being at an event with an ex can be awkward. If she's not that close to your niece, it seems reasonable that she not attend the wedding. You should talk to your niece about it.
My niece always listens to me. That bitch won't be at the wedding.
That is great. If it would be awkward for many people to have your ex at the wedding than it makes sense for them to not be there. Either way, it is good that you talked with your niece about this issue.