Any parent with more than one kid faces this situation on an ongoing basis. I for one, do not agree with you. The invitation was extended to your older D, simply because she is older and good friends with her cousin. She is good company for your sister to take along. It might not be as fun for the older girls if D6 tags along. It might also be more work than your sis had planned on - it is her trip first and foremost. And then - why should D9 miss out.
Your children should always be individuals. Your older D must get plenty of invitations that do not include her younger sibling. And D6's day will come. She will get her special things to do that her older sister will not be invited to.
Being left out or not included is part of life, growing up. Take the opportunity to do something extra special for just you and her. But don't let your older D miss out on this wonderful opportunity.
