will siblings be allowed to go to the kids club together ages 5 and 6?
The answer to this maybe. I can not give you a definitive answer because it will be up to the crew members on board.
When you go to open house on the first day, ask the crew members. Usually they will work with you. I do not know if they will have your 5 year old age up a group or the 6 year old age down a group. That may depend on how close they are to turning 6 and 7 respectively. I would say just be prepared though on the off chance they will not let you.
We have aged our oldest down to the 3-5 year old group a few times because they combined the older groups (6-8 and 9-11) and she did not feel comfortable with the older kids.