It's very hard to advise as we have absolutely no idea why your son's girlfriend is acting like this. And sounds like you don't either.
We don't know why she is so unfriendly with you. We don't know why she is so unfriendly towards your foster daughter (there hasn't been a brief relationship between your son and foster daughter at some point, has there?)
So it's all about trying to second guess her, which never works. Your son is not giving you accurate info either. Claiming it's her house and she can invite who she likes, is what people say to avoid having to explain the truth.
Now this not knowing what is going on with your son's girlfriend has come to a head because of the party.
I really don't know what is best to do about the party because we none of us have the information about what the girlfriend is thinking. Is she being really mean or is there a whole back story we know nothing about?
Can you have a frank talk with son and girlfriend where you ask why they are excluding your foster daughter? Then see how you feel after that about what the right thing is to do about the party.
Until you know why the girlfriend is like she is, you are going to have this kind of issue coming up again regularly. She could just be a very unpleasant person or she could be holding a very longlasting hurt about something. I think you need to know what's behind the behaviour so you can act accordingly in the future eg have as little to do with her as possible/ try to build bridges etc.
And certainly if you do change the party venue etc of the party they are hosting without a discussion involving the girlfriend, I think there's a pretty good chance of it completely ruining any relationship with her anyway. You may decide that is something you can live with, but wouldn't you rather find out first from the horse's mouth what is going on? I don't think you have a lot to lose at this point!