Understanding the Code Logic

First, let's establish the meaning of each symbol based on the provided code:

• $A + B$ means 'A is the husband of B'.
• $A - B$ means 'A is the daughter of B'.
• $A × B$ means 'A is the brother of B'.
• $A ÷ B$ means 'A is the sister of B'.

Decoding the Expression: $S × R - U + N ÷ M$

We will decode the expression step-by-step, determining genders along the way:

1. $N ÷ M$: This means 'N is the sister of M'. From this, we know that N is female.
2. $U + N$: This means 'U is the husband of N'. Since N is female, U must be male. They are a couple.
3. $R - U$: This means 'R is the daughter of U'. From this, we know that R is female. Since U is the father (from step 2) and N is the mother, N is the mother of R.
4. $S × R$: This means 'S is the brother of R'. From this, we know that S is male. Since R's mother is N, S's mother is also N.

Determining the Relationship between S and M

From the steps above:

• We established that N is the mother of S (and R).
• We established that M is the sister of N (from step 1).

Therefore, S is the son of N, and M is the sister of N. This means S is the sister's son in relation to M.