I don't understand how some people are so selfish and so stupid. Their wedding, their choice, has consequences. I know one couple, now married 3 months, who upset a lot of people with Their wedding, their choice and they are now scratching their heads as to why people are not happy with them and no longer wanting to hang out with them. If you want to get along with extended family, you need to pay a little consideration to them.
Also, this blood is thicker than water stupid line. Are some people from a clan and transported back to the 12th Century? Families these days are made up of bio, adopted, and step children. My extended family is. I genuinely feel no different to any of them because of how they arrived here.
I am sensing here that your Step D, or someone advocating for her, doesn't want your DD there as it is her side of the family, and your DD being there may take the shine away from her. Could this be true?
Just wave your DH off with a smile to the wedding and go do something really special with DD. A wedding or major event brings peoples true colours out. They have shown you that they are willing to exclude your DD, and don't see her as family. Well, that works both ways. If your DC is not their family, then that makes them not family to you too. Make sure that for now on your DH pulls the weight for his cousin and wife, and you do nothing for them.