Can i make a will on my property to non-blood related person and what is the amount stamp duty i have ti pay for that

Sir, I have a property (Flat 750 Sq.Ft. with UDS of 550 Sq.Ft. Land in Chennai), I have not married as yet and my present age is 63. I was staying in my cousin's house from my age 19 to 55 along with them and they are also treated me well as their children and one of their family members without showing any differences. During my earning period I was bought the above mentioned property in my name. Now I want to make a WILL jointly in my cousin's third daughter and her 8year old daughter. At present my cousin's third daughter (aged about 33 Years) got legally divorced and did not get re-marriage as yet, but her daughter (8 year girl) being taken care of by father and mother without having any written commitment. At present he (father) also did not get re-marriage. Sir, please suggest, can I make a WILL in my cousin's third daughter and her 8 year old daughter jointly without paying any huge stamp duty (like regular sale deed / gift of property to others). PS:- At present my cousin's third daughter staying with their parents and her parents also surviving day to day life with a pension amount of Rs.39000/- pm. My cousin's daughter does not have any degree and she is working in a private company as an assistant with small remuneration. She did not ask for any compensation from her husband and they mutually agreed and divorced. Please give me your best advise sir.