In Japanese, how do I refer to my mother's cousin? She and my mother share the same great-grandparents. My mother's cousin's mother was the older sister of my mother's father (my maternal grandfather). I think the English term for my relationship to her is that she is my first cousin once removed (my mother's cousin has no children, whereas many websites about calculating cousin relations tend to want you to start from your generation and move outward [i.e., my second cousin's mother is...]). I think if she had had children, they would be my second cousins. Is this correct?
I am writing my first email to my mother's cousin's Japanese relative and I do not know in what way the two of them are related (whether on her father's side, meaning the addressee is also on my side of the family, or on her mother's side, in which case the addressee is from the opposite side of her family tree), but I want her to easily understand what my relation is to my mother's cousin.