My little brother, who is now 26 years old, is driving my parents mad. He is diabetic since he was 15 years old. He smokes, drinks, does not work or study. He takes money from my parents.... etc. He makes us all miserable, especially my parents. Above all, he has a big knowledge of Islam. I still like my brother after all the bad things he is up to, but I do not know what to do. I wish I could find a cure to save him. Can you please help me?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

We advise you to spare no effort to save your brother and try your best to be the cause of his guidance. Be sincere with him, show him affection, and desire good for him. To be successful in this mission we advise you to do the following:
First, try your best to advise him in private since advising in public causes much embarrassment.
Second, show him your affection and your desire to help him.
Third, threaten him of the bad consequences of his deeds in this world and tell that the disease that afflict him might be the result of his bad deeds. Remind him that such deeds lead to severe punishment in the Hereafter.
Fourth, choose a suitable time to advise him, especially on the occurrence of incidents such as the death of a relative, or friend and the like.
Fifth, make sincere Du'a for him and also ask your parents to make Du'a for him since the Du'a of parents for their children is surely responded to.
Allah knows best.