My grandchildren call my sister by her first name, as do my children. Same for my sister-in-law. When they're old enough (roundabout now for the children, now they're 40!) I have explained the arious relationships.
Sorry if this sounds sexist, but boys are generally not interested in family relationships - up until now, mine have just about managed to get their heads around uncles and cousins. Their wives, on the other hand, remember all our obscure relatives, their birthdays and illnesses,pets and interconnections. I sometimes feel I'm talking to a brick wall when I tell DH about any of my cousin', let alone the next gener.
So, IF I had any great niblings, I would sign with my first name. The "your" in the OP's case is fine, as it distinguishes great-auntie Jane from "Auntie Jane. who is great"!