All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
There are many Ahaadeeth indicating that the one who gives is better than the one who receives. Hakeem ibn Hizaam narrated that the Prophet said: “The upper hand (that gives) is better than the lower hand (that receives); and begin (charity) with those who are under your care; and the best charity is in times of surplus; and he who asks (Allaah) to help him abstain from the unlawful and the forbidden, Allaah will fulfill his wish; and he who seeks self-sufficiency will be made self-sufficient by Allaah.” [Al-Bukhari and Muslim]
If your brother is as you have described, then there is no doubt that he is making a mistake. It is his duty to relieve his parents of the burden of supporting him; they strove in raising him and taking good care of him for long. He should be dutiful towards them and provide for them instead of being dependent on them like that. Please refer to Fatwa questions 82254 and 84529.
Our first word of advice to you is to have patience and pray to Allaah to guide him to the right path and help him come to his senses. He should be given advice kindly, and you should choose the suitable times when he is likely to accept the advice. You can seek the help of those whom he respects among your relatives and friends.
There is no harm in discussing the situation with your parents; perhaps they have over-pampered him, thus leading him to have such a negative attitude. They may need to be more firm in bringing him to his senses. However, you should be careful not to talk harshly to your parents for this is considered undutifulness. Allaah, The Exalted, enjoined us to be kind and gentle with them; this is a manifestation of the rightful dutifulness towards them. Allaah, The Exalted, says (what means): {And your Lord has decreed that you not worship except Him, and to parents, good treatment. Whether one or both of them reach old age [while] with you, say not to them [so much as], "uff," and do not repel them but speak to them a noble word.* And lower to them the wing of humility out of mercy and say, “My Lord, have mercy upon them as they brought me up [when I was] small.”} [Quran 17:23-24]
However, if your parents willingly accept this situation and are not bothered by your brother’s dependency on them, then it is up to them; as this may be a source of happiness for them. In this case, you should have patience and accept the situation. You have the right, though, to refuse to provide for your brother and his family. We also advise you to continue advising him kindly in this regard as long as there is hope for correcting the situation.
Allaah Knows best.