How do I deal with my brother who mistreats my parents? Should I hit him or abandon him?

The rights of parents are great, and honoring them is among the most obligatory duties, while disobeying them is one of the greatest major sins. Part of honoring them is addressing them with politeness, gentleness, and humility, as Al-Qurtubi explained the verse: "And speak to them a noble word" (Quran 17:23) to mean the speech of a sinful servant to a harsh, stern master, i.e., that the child should humble himself before his parents. If a brother mistreats his parents, he must be forbidden from this wrongdoing and reminded of his parents' rights over him. It is not permissible to beat or punish him, but it is permissible to shun him if shunning benefits him and deters him from the wrongdoing. The believer should be merciful towards sinners and hope for their guidance and repentance. This does not contradict forbidding evil, because the hatred is for the sin, not the sinner. The fundamental principle in forbidding evil is to do so with gentleness and forbearance, for Allah is gentle and loves gentleness.