AFTER READING MORE RESPONSES:
There is no legitimate school system in the United States that can force you to invite children or adults to your home for a party. If any U.S. school board has such a charter, I can assure you it is not in line with our "freedom of choice", and probably should be amended.
I'm sure your intention is to give your child a nice party and not to hurt the feelings of any little child. If that little girl's biggest problem is not getting to attend an ALL boys party, she will be just fine. If she is upset (not heartbroken), it is up the HER mother, to explain and comfort her.
I wish your twins a very happy birthday and hope your don't feel forced to go to the additional expense and work by inviting extra children (girls) that your twins don't want to be at THEIR party, for one child's, temporary disappointment.
S.,
You should let the mom of the twins know this party is just for BOYS! There will come a time when a mixed group will be fine, but not this time. You can't let others dictate what you chose to do in your own home.
It would be up to the twins mother to do something speical for her daughter and explain to her that she will NOT always be included in her brothers activities nor he in HERS!
Blessings.....