The ages groups are 12 to 14 and 15 to 17 so there is a chance that your younger sister wont be allowed in the older club.
However, 2 years ago I was on the Spleandour of the Seas and there were people in the older teen club who were 13/14 and no one checked. But when I went on my first cruise in 2002, they checked me and my friends ages. I was old enough but they wouldnt let my friend in.
Sometimes they check ages and sometimes they dont. Best of luck.