Siblings using the skills with each other

Hello Nicholeen, it's Dawn Pilgrim. If one child wants to come in on a game that two siblings are playing, but the other children don't want her to, would the 2 children give a no answer, or would they ask to disagree appropriately, or would they let her in?
How do I ballance the need of the (younger) child to accept a "no" answer with the need of the older two to include their younger sister and be kind. (Younger one is quite dominant and demanding).