The correct option is Sibling.

Let us break down the given family relationships step-by-step to determine how A is related to B:

1. "A's father's wife": A person's father's wife is their mother.
So, A's father's wife = A's mother.

2. "A's mother's only daughter": The only daughter of A's mother is A's sister (or A herself, if A is female). In any case, A and this daughter share the same parents.

3. "is B": Therefore, B is the daughter of A's parents.

Since A and B share the same parents, A and B are brothers/sisters to each other, which means A is a Sibling of B.