Land Sale after Mother's Death

I am the only child (son) of my parents. My mother passed away when I was in 5th Grade. Also my dad went through heart surgery when I was in college and since then we have a home nurse to take care of him. She is there with us for almost 20 yrs now. Since my college and work has always been outside Kerala, my dad and home nurse lived in a house (house is my dad's property) in kerala. We also have my mother's land (which she got due to death of her parents). We want to make an legal agreement that after my dad's time, a small portion of my mother's land will go to our home nurse (as she wants written assurance that she will be compensated). All this while she was like a member of the family and there has not been a formal salary given to her but we have helped financially on various critical situations in her life. Want your advice on how to go about forming this document which ideally should say something like "if you take care of my dad till his time, you will get say 2 cents of my mothers property after his death). This agreement should be formed such that it later should not stand in my way if I want to sell my share of the property (total - 2 cents).