A goes on a picnic and meets a woman B who is the sister of A's wife. How is B…

Concept: In reasoning-based blood-relation problems, a relative connected to a person only through that person's spouse (not by blood) is named using the suffix "-in-law", matched to the relative's gender: a spouse's brother is a brother-in-law, and a spouse's sister is a sister-in-law.

1. A has a wife, since the question refers to "A's wife".

2. B is described as a woman who is "the sister of A's wife" — that is, B is the sibling of A's spouse.

3. A relative reached only through marriage (not blood) takes the "-in-law" suffix matched to gender. B is female and is the spouse's sibling, so B is A's sister-in-law.

Cross-check: Checking the relation the other way: A would be B's brother-in-law (A is the husband of B's sister) — the two labels are consistent, confirming there is no direct blood link and the gender-matched "-in-law" label is correct.

Result: So, B is related to A as his sister-in-law.