Does the asker sin if he refuses to give his brother money when asked, while reminding him of the monthly assistance, and is reminding him of that considered a favor that invalidates the reward, knowing that the brother is a heavy smoker and the asker is saving for marriage?

You are permitted not to lend money to your brother, especially if doing so would motivate him to seek employment. Lending is a recommended act, not an obligatory one. It is commendable to advise a brother who is not working to strive in seeking provision. It is better for this advice to come from someone else whose words he accepts. As for reminding him of what you give him, this falls under forbidden obligation (manna), unless there is a genuine necessity that calls for it. It is permissible to give charity to a smoker as long as it is not certain that he will use the money for disobedience.