Help! My Dad feels my step-mom is planning to divorce him now that he's retired and in bad health.?

My Dad wants me to help him protect his ets in case my stepmom divorces him. He has savings in the bank plus retirement funds through his last employer. My Dad is my stepmom's third husband. When she divorced her first two husbands she was able to take some of their ests. My Dad feels she and her three sons from her first marriage just want his money but my Dad wants to leave something for my sister and me. What is the best way for my Dad to protect his ets from my stepmom and her three sons in the event she divorces him? Please help, thanks!