I'm going against the grain here.
The parents of twins make a huge effort to make sure their children are treated as individuals, encouraged to make their own friends, have their own special occasions, etc.
In this case the DSD has all of her 'own' things at her DM's house, and share things at her DF's house.
The DD on the other hand doesn't have any of her own things. Whatever she has, she shares with her DSD whenever they are there.
I don't think its unreasonable for her to have her 'own' party with her own school friends. Her DSD is NOT a school friend, she goes to a completely different school. If they had gone to the same school this would be a very different situation.
The timing, however, is unfortunate because it will be on a weekend when DSD is there, but that's just the way it is.
I don't think it's wrong for your DD to have something just for herself occasionally. All children need to.
In fact I would recommend you trying to find a way of making sure your DD has her own room that she doesn't have to share with her DSDs if at all possible, obviously DSD will need some space too though. Hard to afford somewhere with enough rooms for everyone to have space though.