My daughter is six and I often get invitations from her friends that specifically spell out "No siblings please! Space is limited!" Usually that's when it's at a gym or a specific place that costs more per person. My son is close in age and doesn't like to miss out so in certain certain circumstances if the party is at a place he likes like Chucky Cheese's, then I may ask if he can come and I offer to pay for him. But if you're having a party specifically geared towards 9 year olds, it's seems perfectly acceptable to just invite the one friend who is the same age, not the sisters, older or younger. If the parent asks you, then you can decide, but I don't think you should feel that you have to invite them or that they're expecting to go. Have fun!