If your aunt is my grandmother, what relation are you to me? Naming relations is a tricky business. Think about the offside rule in football for a moment. If you overlook the recently-introduced subjective elements about “interfering with play”, it’s actually pretty simple. “Explained” by several people gesturing and talking simultaneously, using peanuts and pint pots on a wobbly pub table, voices raised over the background noise, it becomes fiendishly difficult to transmit to a newbie. I’ve never had them explained using snacks or shouting, but I’m sure that over my lifetime I’ve had the concepts of “first cousin” or “once removed” laid out for me several times. Usually this happens at a stand-up gathering without access to paper and pencils. The concepts have never stuck, and I’m actually pretty sure I’ve heard several incorrect definitions floating about.
I think the/a correct answer to my opening question is “I am your first cousin, once removed”. My great-grandparents are your grandparents, even if your aunt (my granny) might not be a blood relation of yours.
My brother proposes an alternative system. His way, two legitimate answers could be “father-mother-brother-daughter” and “mother-mother-sister-son”. All solutions go up the family tree, up again, sideways, and then down - and I think all would conventionally be called “first cousin, once removed”. In “the Graham system” there is no ambiguity - but it isn’t concise.
Anyway, all this came to mind because I saw this Chart of Cousins, which contains a great mathematical explanation from wikipedia. Recommended reading if your mind works like mine - which I will admit is not how everybody’s works. Sometimes I like to work things out for myself!