My first post stands, as I have sympathy for OP, her DP and his DD...however, I just read the update and saw that OP doesn't live w/ DP. This changes things a bit, IME. Plenty of weddings don't allow you to bring plus one, rather they address to two cohabiting life partners or 2 spouses.
Also, I feel there is some missing element here. Are the emotions of DD coming on strong lately (normal in the lead up to wedding) about the loss of her mother and aunt/uncle reacted to something DD said - maybe DD said something not unkind toward OP, but possibly that she had always wished both her parents would be at her wedding. In overreaction, auntie/uncle decide to smooth the waves, ask their BIL why his girlfriend, who isn't even live-in, will be there?
Or, DP very well may not be sharing the full story here. Maybe DP wants to preserve DD's emotions and changes his mind last minute and it's useful to use the aunt/uncle as cover? He may love OP but knows this one instance could be painful to have her along. He didn't divorce his wife, she died.
OP, unless the family have met you and decided they didn't like you, I would do your best to let this one go. It's likely just a VERY charged lead-up to a wedding day and DD's DM being gone is overwhelming to them all. Go have fun on your spa day.