When my nephews and nieces started having children I chose to be referred to as Grand Aunty and have used that for almost thirty years.
The grand-child of my eldest niece is my great-grand-niece, so I'm known as great-grand-aunty to her.
My logic is that I have grand-parents and great-grand-parents, I don't have great-parents and great-great-parents so using grand makes sense to me - and I rather like being called a grand aunty :-)
