Hi. I don’t think you should ask about it. You were denied a plus one. Maybe many people/you, are/were because it is a small wedding. Or a cost factor etc. It may not be because of your particular date/BF. If you ask and they say they don’t want him there, you will just be more upset and angry. Or if they offer other reasons, you may not believe them. You seem defensive about him. Are there good reasons they don’t like him?
I think you should forget this side issue and just decide if you want to spend the time and money to support, see your “niece” get married. If you can go and not be resentful, go and enjoy the event.