The parents may have excuses for this mistreatment. They may be strict with one son than the other for more than one reason. He may be older than the other sons and his mistakes should not be overlooked. They may want to discipline him to observe good conduct to make him a good example to his younger brothers. It is impermissible for the child to be undutiful to his parents even if they mistreat him. He should follow Allah's saying,
Repel evil with that which is better.
Parents should be the most favored with well treatment, as Allah (Exalted be He) says,
And that you be dutiful to your parents.
...but if they strive to make you join with Me (in worship) anything (as a partner) of which you have no knowledge, then obey them not. Unto Me is your return and I shall tell you what you used to do.
(Part No. 25; Page No. 125) It is obligatory upon the son to show kindness and obedience to his parents, even if they endeavor to make him associate with Allah another partner, which is the gravest major sin. Allah orders him to follow the straight path as He says that everyone will be punished or rewarded on the Day of Resurrection. The Prophet (peace be upon him) has ordered us to be grateful to our parents and warned of being undutiful to them, as this act is one of the gravest major sins. Your regret for your disputes and cutting ties with them and the Sadaqah given by you on their behalf are appreciated. Had you declared this act to your parents, peace and harmony among you would have been achieved. I hope Allah will forgive your sins and theirs and reward you and them for the Sadaqah as He multiplies the reward.
With regard to the parents, they have to treat their children justly. The Prophet (peace be upon him) said,
Fear Allah, and be just to your children.
May Allah grant us success. May peace and blessings of Allah be upon our Prophet Muhammad, his family and Companions!