Rita told Mani, “The girl I met yesterday at the beach was the youngest…

In a blood-relation puzzle, fix one person as the reference point and decode every stated family term one link at a time by building a family tree outward from them. A person's ‘brother-in-law’ is either the husband of their sister or the brother of their spouse — either way, someone of that person's own generation who joined the family by marriage. Once every stated link is placed on the tree, the relationship the question asks about can be read directly off it.

1. Fix Rita's friend as the reference person and call her mother M.

2. ‘The brother-in-law of my friend's mother’ is M's brother-in-law — a man of M's own generation, so he is an uncle (by marriage) of Rita's friend.

3. That uncle's ‘youngest daughter’ is a child of Rita's friend's uncle, which places her in the same generation as Rita's friend — one generation below M.

4. A parent's sibling's (or sibling-in-law's) daughter is, by definition, a cousin.

Checking the other options against the same tree rules them out: she cannot be Rita's friend's own daughter (that needs Rita's friend herself to be a parent, not an uncle); ‘Friend’ ignores the family tree the sentence is building; and she cannot be an aunt (an aunt is a generation above Rita's friend, while this girl shares Rita's friend's generation).

So the girl is Rita's friend's cousin.