I would bet Great Uncle-in-law if you want pedantry.
a Great-uncle is your parent's uncle. Or as Wiki puts it "A Great-uncle is the male sibling of one's grandparent."
There is no term "Grand Uncle" which would be more logical.
And the adjective for uncle-like behavour is avuncular - and there isn't a similar one for Aunt, but if it is also avuncular I ain't never seen it used so.

& FWIW

cousins are 1st, 2nd, 3rd etc if they are both the same generationally. And once, twice etc removed if there is a generation imballance.

Now does a female Morris side regard themselves as step sisters?