Is a mother considered disobedient (nashiz) for leaving the house without her husband's permission and refusing to sleep with him due to his sexual impotence, knowing that she serves him and fulfills his other requests? Does this nullify her right to maintenance for clothing and medicine despite her illness, and what is the current status of their marriage?

What you have mentioned about your parents, concerning their exchange of insults and curses for the slightest reasons, their disputes, and the severance of their relationship, contradicts the affection and mercy obligatory between spouses, as stated in His (Allah’s) saying: "And of His signs is that He created for you from yourselves mates that you may find tranquility in them; and He placed between you affection and mercy. Indeed in that are signs for a people who give thought." And your mother's leaving the house without your father's permission and her refusal to spend the night with him under flimsy pretexts is considered nushuz (rebellion/disobedience), because nushuz is a wife's disobedience to her husband and her defiance of his rightful authority. If the mother's nushuz is proven, she has no right to maintenance, clothing, or any other marital rights from the father. Her husband's sexual impotence does not intercede for her nushuz, because he can still enjoy his wife in ways other than sexual intercourse. If the impotence occurs after one act of intercourse, the wife has no option to annul the marriage. Your father is still your mother's husband as long as he has not divorced her, and you must strive to reconcile them.