Will the eldest brother be punished for not gathering his siblings who were raised by relatives, especially since his financial situation is weak and his stepmother is harsh? And what if his siblings refuse to come and live with him?

You are not held accountable, Islamically, for not gathering your siblings in one house, as long as they have someone to take care of their affairs. However, if they do not find anyone, or if there is a fear of them being lost due to poverty or inability to earn a living, then you are obligated to provide for them and secure housing for them, provided that there is something left over after the expenses for yourself, your wife, and your children.