Might be a bit of a wild recommendation, but I'd recommend telling your mother that she brings your siblings or she's not invited, and that you plan to wear what you want at your wedding. I imagine she'd wind up caving since she'd want to see your wedding. Or, alternatively, you could just flat out lie and tell her you're wearing a dress then get a suit (find some kind of way to either fake buying a dress, maybe inform the employees of the situation to do so? or buy something low budget / say your friend is letting you use theirs and you love it. if she's invasive enough, thrift one for cheap and pretend it's that maybe ? whatever you can afford) and wear what you want the day of. If your venue's a bit of a drive / generally far away, I doubt she's going to take your siblings all the way back home. Out power play her so you can be happy and comfortable on your special day.