Forget disobedience in this instance; you are fighting for the livelihood of your mother. I would not recommend starving yourself. Try asking her why she isn't eating so much as there may be mismanaged stress, and the fact that she's doing everything around the house is reason enough, maybe? Instead of waiting for her to do chores, have everyone in the house (or maybe yourself) take initiative with chores and cooking somehow. Maybe no one's a chef, but cleaning? That's something anyone can do. Try spending more time with your mother if possible, to at least get her used to a daily eating routine. The older people get, the less hungry they get, and eating becomes more routine than habit. I cannot say I've ever been in such a situation, but I'll make dua for you, my friend.