What is the legal ruling regarding a mother obliging her eldest son to financially support his married siblings, threatening him with divine wrath and punishment if he does not comply, especially when they constantly demand money from him and disparage what he gives?

Scholars differ on the obligation of financial support (nafaqah) for siblings. The Malikis and Shafi'is hold that it is never obligatory, while the Hanafis deem it obligatory. The Hanbalis stipulate that it is obligatory if the provider will inherit from them.

According to the view that financial support is obligatory, three conditions must be met: 1. They must be poor, having no wealth or earnings to suffice them. 2. The provider must have more than enough for their own expenses and those they are already supporting. 3. They must not have an obligatory financial support due from another person, such as a husband.

Based on this, you are not obligated to provide financial support for your brothers, especially if they are wealthy or capable of earning, nor for your married sisters. Obligatory financial support is limited to basic needs.

Your abstention from providing financial support in this situation does not constitute disobedience to your mother. If your siblings treat you badly despite your kindness, this will only increase your reward with Allah.