A man, pointing to a photograph, says, "The lady in the photograph is my nephew's maternal grandmother." How is the lady in the photograph related to the man's sister, who has no other sister?

Introduction / Context:
This blood relation puzzle involves a man, his nephew and a lady in a photograph described as the nephew's maternal grandmother. The question then asks how this lady is related to the man's sister, given that he has only one sister. The chain looks a bit intricate, but once you remember that maternal grandmother is the mother of someone's mother, the connections become much easier to follow.

Given Data / Assumptions:
- The speaker is a man. - The lady in the photograph is the man's nephew's maternal grandmother. - The man has one sister and no other sister. - Nephew means a son of the man's brother or sister. - Maternal grandmother means the mother of the nephew's mother. - We assume standard family structure without adoptions or unusual uses of terms.

Concept / Approach:
First identify who the nephew is likely to be the child of. Since the man has exactly one sister, the most natural and exam-friendly assumption is that the nephew is his sister's son. Under that assumption, the nephew's mother is the man's sister, and the nephew's maternal grandmother is therefore the mother of that sister. The question then becomes: how is the lady, as mother of the sister, related to the sister herself?

Step-by-Step Solution:
Step 1: The nephew is the son of one of the man's siblings. Since the man has only one sister, a common exam assumption is that the nephew is the sister's son. Step 2: If the nephew is the son of the man's sister, then the nephew's mother is that sister. Step 3: Maternal grandmother of the nephew is the mother of his mother, that is, the mother of the man's sister. Step 4: The lady in the photograph is that maternal grandmother, so she is the mother of the man's sister. Step 5: Therefore, from the viewpoint of the man's sister, the lady is her mother.

Verification / Alternative check:
Draw a simple tree: put the man and his sister on one level, their mother above, and the sister's child (nephew) below. Label the mother as the lady in the photograph. It is clear that she is the maternal grandmother of the nephew and simultaneously the mother of the sister. This visual check confirms the verbal reasoning.

Why Other Options Are Wrong:
- Cousin: A cousin is another grandchild of the same grandparents, not a grandparent themselves. - Sister-in-law: That would be the wife of a brother or the sister of a spouse, not the sister's own mother. - Mother-in-law: This would be the mother of the sister's husband, a different person altogether. - Aunt: An aunt is a sister of a parent, not the parent herself.

Common Pitfalls:
Learners sometimes assume that nephew must be the son of the man's brother, which introduces extra cases and confusion. In standard exam-style questions, when a unique sister is mentioned and no brother is discussed, the nephew is usually taken to be her child. Another frequent mistake is to confuse maternal grandmother with paternal grandmother. Remember that maternal always refers to the mother's side.

Final Answer:
The lady in the photograph is the man's sister's mother.