Is it permissible to advise one's mother to reduce her voluntary fasting when it causes her fatigue and illness? Is it permissible for her to pray sitting due to her illness, specifically knee pain? And would she be sinful if she prayed standing despite her illness, believing that doing so is better?

There is no blame upon you for advising your mother gently and kindly to reduce her fasting if you believe it causes her fatigue. If she responds, then all the better; otherwise, do not cause her distress with excessive advice. There is no blame upon her for praying while sitting if she is unable to stand or if it is difficult for her to do so, due to the Prophet's (peace be upon him) saying to Imran ibn Husayn (may Allah be pleased with him): "Pray standing; if you are unable, then sitting; and if you are unable, then on your side." There is no sin upon her if she forces herself to stand during prayer, as long as it does not cause her harm.