Introducing a boy, a girl says, "He is the son of the sister of my mother brother." How is the boy related to the girl?

Introduction / Context:
This question describes a boy using a slightly complicated phrase: "He is the son of the sister of my mother brother." You must determine how this boy is related to the girl who is speaking. The puzzle is based around the concept of maternal uncles and aunts and their children.

Given Data / Assumptions:

• The speaker is a girl.
• She refers to "my mother brother", which is her maternal uncle.
• The phrase "the sister of my mother brother" points to a female sibling of that uncle.
• In typical exam style, this sister is taken to be an aunt, a different person from the girl mother.
• The boy is the son of that aunt, that is, the son of the sister of the girl maternal uncle.
• We assume a normal family where children of aunts and uncles are cousins.

Concept / Approach:
The key is to recognise that "my mother brother" is a maternal uncle and that his sister is a maternal aunt. The boy is the son of this maternal aunt. Children of maternal aunts and maternal uncles are cousins of the girl. Therefore, the boy and the girl are cousins.

Step-by-Step Solution:
Step 1: Identify "my mother brother". This is the brother of the girl mother and is called her maternal uncle.Step 2: Identify "the sister of my mother brother". This is a female sibling of that uncle. In exam context, this is treated as a maternal aunt of the girl and is taken as a distinct person from the girl mother.Step 3: The boy is the son of that sister, so he is the son of the girl maternal aunt.Step 4: The girl is the daughter of her own mother, who is a sibling of the maternal uncle and the maternal aunt, so the aunt is her mother sister.Step 5: Therefore, the girl and the boy are children of sisters or, more generally, children of siblings in the same generation.Step 6: Children of siblings are first cousins to each other.Step 7: Thus the boy is a cousin of the girl, and the girl is a cousin of the boy.

Verification / Alternative check:
Use names for clarity. Let the girl mother be Meera and her brother be Mahesh. Mahesh is "my mother brother". Mahesh has a sister Anita, who is another daughter of the same grandparents and is not Meera in this exam interpretation. Anita is therefore the girl maternal aunt. If Anita has a son Ravi, then Ravi is the son of the sister of Meera brother. Ravi and the girl are children of two sisters, Meera and Anita, so Ravi is the girl cousin.

Why Other Options Are Wrong:
The boy is not a brother-in-law, because that would require a marriage relation that is not mentioned. He is not a father-in-law as he is clearly in the same generation as the girl. The word niece describes a younger female relative of an aunt or uncle and does not fit a male child. Uncle also does not fit, because uncles belong to a generation older than the girl.

Common Pitfalls:
Some learners think that the sister of the girl maternal uncle must be the girl own mother, which would then make the boy her brother. However, the question provides options that do not include brother, and typical exam interpretation treats the mentioned sister as another aunt, not the mother. It is important to follow the structure suggested by the answer choices and to recognise that aunts and uncles can have multiple siblings.

Final Answer:
The boy is related to the girl as her cousin.