Just ask? Isn't her older sister in the next group? And she's at the top of the age range, so it might not be as big of an ask as it may feel. Lots of parents want both kids together in these kinds of orientation activities, and it's also not that unusual for younger siblings to be a little precocious socially and emotionally, compared to same-age firstborns. I don't think you even need to get into her grade placement, actually.

...pronounced like the long vowel and first letter of the alphabet...