All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Firstly, we would like to commend your keenness on doing what is good. You should know that your mother is not obliged to serve your grandparents; however, she earns great rewards for doing so as underlined in fatwa 84909. You should advise her to be patient and remind her of the great reward that Allaah shall grant her for doing so, because serving them pleases her husband and helps her show kindness to two old Muslim people as well as exert efforts to benefit Muslims.
Jaabir married a previously married woman to help him take care of his seven sisters and serve them. She did do so although she was not obliged to do it out of her obedience to her husband as a form of good companionship. Faatimah used to grind grains with the grindstone herself. Asmaa' used to fetch fodder for her husband's, Az-Zubayr, horse from a distant place. Many scholars underlined that the woman is not obliged to carry out such chores; however, the women of the righteous predecessors willingly carried out such chores, hoping for the reward of Allaah.
You can also advise your father to relieve your mother of some of these duties; he should hire someone to help her with the household chores and serving his parents if he can afford it. He can also help her himself or have you help her carry out those chores. Indeed, all difficult matters are made easy by means of good and compassionate words and kindness. We ask Allaah to reconcile between them and bring their hearts together.
Allaah knows best.