UK visitor visa for unemployed parents

Hi All,
Please help me to file my parents visitor visa to UK from India.
My husband will be sponsoring their UK visit for 2weeks. My Husbands documents provided -
Employment letter
Bank statement
Pay slips
House Rent Agreement
Utility Bill

My father and mother used to run a road side food joint in a small village in India till 2015. They have Trade liscence document till 2015. Currently they are unemployed and my father age is 60. They have their fixed deposits in bank and live with my younger sister and grand ma. My sister is working and runs the house expenses and my parents are the sole caretaker of my grandmother.
When my parents will be in UK for 2weeks one of our distant relatives will take care of our grandma.
Please suggest what documents to be provided to support my parents visa.
They have confirmed and return air tickets.