Blood Relations
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How to solve blood-relation questions
Symmetry pairs
\text{father} \leftrightarrow \text{son},\ \text{mother} \leftrightarrow \text{daughter},\ \text{uncle} \leftrightarrow \text{nephew}
Every relation has a mirror: if A is B's uncle, B is A's nephew (or niece).
In-law map
\text{spouse of child} \to \text{son/daughter-in-law};\ \text{parent of spouse} \to \text{father/mother-in-law}
Sibling of spouse, or spouse of sibling → brother/sister-in-law.
Cousin
\text{cousin} = \text{child of a parent's sibling}
Aunt/uncle's children are cousins — never 'nephew'.