Will the father be held accountable for abandoning his first wife and cutting off her expenses because of her refusal to return home and his poor financial circumstances, and is the wife wronged in this situation, and is the questioner obligated to support her stepmother despite her responsibility for her family's expenses and her impending marriage?

A woman is not permitted to leave her husband's house without his permission. What you mentioned about her attachment to her grandchildren and so on does not justify her departure. By doing so, she is considered disobedient (nashiz), and she forfeits her right to maintenance from her husband. If she returns to obedience, then her maintenance becomes incumbent upon whoever is responsible for her upkeep, namely the son and his brother. Efforts should be made to reconcile them, reminding her of the obligation to obey her husband and the prohibition of disobedience, and seeking his forgiveness for her.