Is it obligatory to obey one's parents in not filing a lawsuit against an oppressive brother, given the resulting loss of material and moral rights?

If you find no way to reclaim your right except by suing your brother, there is no sin upon you in doing so, even if your parents become angry; for it is not their right to condone his injustice or to compel you to relinquish your right. Disobedience (al-uquq) that is forbidden is that which harms the parents due to the child's transgression, not his demanding his right. If it is permissible to sue one of the parents to demand a right, then suing a brother is even more permissible. However, if you are patient and forgive him, seeking the pleasure of your parents, then you will have a great reward.