X is the brother of U. U is the daughter of Z. Y is the daughter-in-law of Z. V is the son of X. Who is the mother of V?

Let's trace the family relationships step by step:
1. X is the brother of U ($\text{X}(+)$ and $\text{U}$ are siblings).
2. U is the daughter of Z ($\text{U}(-)$ is Z's daughter, which means X is Z's son).
3. Y is the daughter-in-law of Z. Since X is the son of Z, Y must be married to X ($\text{Y}(-)$ is X's wife).
4. V is the son of X ($\text{V}(+)$ is X's son).

Since Y is the wife of X, she is the mother of X's son, V. Thus, Y is the mother of V.