My mother is extremely sick and complains a lot.

Answer

Be of service to her to the best of your ability. Tell her that it comes in the Hadith that sickness is a purifier. It cleanses us from all our sins provided that we do not complain. Complaining will not cure the sickness. On the contrary the sickness will become worse. Sickness is also a ni'mat (favour) of Allah Ta'ala. He purifies us here on earth with sicknesses and other difficulties so that we can enter Jannat fully purified. Advise your mother to keep her tongue constantly engaged in Thikrullaah. Maut can come at any moment. Therefore, the tongue should always be engaged in some Thikr to ensure departure from this dunya with the Kalimah on the tongue.