Hey tink,
I asked my DD the CM (Youth Counselor in the Club) this question for someone else last week. She said that older "siblings" (relatives probably count) CAN come into the Club with younger siblings. Since the Club age groups are 3-4 and 5-7, the activities might be a little juvenile for a 9 year old but if you check at the desk the CM counselors could help you choose some activities that the older one could participate in with her younger cousin.
Disney's all about helping guests have a Magical experience.