co-great-nibling-in-law

The grandchild-in-law of one's spouse's sibling.

My co-great-nibling-in-law is a good person.

great-nibling-in-law

1. The grandchild-in-law of one's sibling.
2. The grandchild of one's spouse's sibling.

My great-nibling-in-law is a good person.