An uncle is a parent's brother. An aunt is someone who marries that uncle. Or vice versa.
Okay, so she's not your parents sibling, but instead married to your parents sibling.
But she's still your cousin? A blood relative?
So she is still the granddaughter of one of your grandparents, provided you mean first cousin.
This still makes no sense.
I'm sure there is inbreeding going on here somewhere.
Could you draw me a family tree?