I am somehow expecting the uncle to spill the beans somehow. I know it wouldnt happen but...
It can happen. All it takes is mentioning Mitsuki in a conversation and he’ll use “her”, which will startle Aya.
“Her? Who?”
“…my niece, duh” [points at store clerk]
Idk how japanese can do it without using the equivalent of “she”.
In Japanese it is very easy and natural to not use pronouns in conversations, so it is not hard of uncle to just never use "she". However, the most likely alternatives would be her first name (as he is a family member) or "niece", both of which are gendered (at least I think 'Mitsuku" is female only, but not too sure). So the end result would be the same.