How is my mother's sister's brother's wife's child related to me?

The answer is a cousin. This may be a tricky question but when you look it in one by one you can answer it correctly. Look and imagine your family tree first in the first two words which are mother’s sister which is technically your aunt. Then proceed to the next while carrying your aunt in the scenario. Your aunt’s brother is your uncle.

Following the previous step, your uncle’s wife is your aunt by law, and their child is your cousin. A cousin is a familial relationship that is two or more generations away from you having a known common ancestor. So next time you encounter a question like this, answer one relationship at a time.