Let's analyze the information step-by-step:

1. b is the father of q.
   • So, b → father of q
2. b has only two children.
   • b has exactly two children.
3. q is the brother of r.
   • q and r are siblings (brothers/sisters), and q is male.
4. r is the daughter of p.
   • r is female and child of p.
5. a is the granddaughter of p.
   • a is female, and p is her grandparent.
6. s is the father of a.
   • s → father of a.

Step-by-step deduction:

• Since r is the daughter of p, and q is r's brother, q is also a child of p.
• b is the father of q and has only two children: q and r.
• Therefore, b is the father of both q and r.
• Since r is the daughter of p, and b is the father of r, p must be the mother of r (and q).
• So, p and b are the parents of q and r.
• a is the granddaughter of p, and s is the father of a.
• Since a is the granddaughter of p, and s is the father of a, s must be the son of p or the son-in-law of p.
• But since b and p are parents of q and r, and a is the granddaughter of p, a must be the child of either q or r.
• Since a is the granddaughter of p, and s is the father of a, s must be married to either q or r.
• q is male, r is female.
• s is the father of a, so s is the husband of either q or r.
• Since q is male, s cannot be q's husband.
• Therefore, s is the husband of r (female child of p and b).
• So, s is the son-in-law of p.
• q is the brother of r.
• s is the husband of r.
• Therefore, s is the brother-in-law of q.

Final answer:

s is the brother-in-law of q.