easy
Your mother's brother's only brother-in-law is asleep on your coach. Who is asleep on your couch?
Common Wrong Answers
“Your uncle”
Your uncle is your mother's brother, but the riddle specifically asks for the only brother-in-law, which refers to your dad.
“Your brother”
Your brother is not the brother-in-law of your mother; he would be a sibling instead.
“Your grandfather”
Your grandfather is related to your mother but is not her brother-in-law; the riddle is looking for your dad.