the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
You may stop helping your mother-in-law if circumstances are as you have mentioned them in your question. You will not be sinful for doing so. However, you must advise her close relatives to take care of her.
Please keep in mind that you will be rewarded for everything you do for her. Helping her is not obligatory upon you. It is a favor from you, and is an act of charity. Doing such good deeds and bearing her ill treatment patiently could be a golden oppurtunity to attain Allah's pleasure, especially if you continue calling her to Islam.
Source: Islam Today