Explanation:

Let’s consider the two statements one by one.
Statement 1:
X is the brother of Y, and Y is the brother of Z.
But we do not know whether Z is a male or a female. So, we cannot say whether Z is the brother or sister of X.
Statement 2:
X, Y, and Z are siblings. It tells us nothing much. Here, also gender of Z is not known. So, Z can be either brother or sister of X.
Even if we combine the two statements, we cannot answer the given questions.