Gift Deed From Mother To Son

My father expired in 2008 leaving a plot of 274 yds, Me, my mother, sister, grandfather and grandmother were his legal heirs, In 2010, 4 of us executed a "Deed of Relinquishment over joint property" to my mother. Later, we built a 4 storied house on the property taking a housing loan as co-borrowers (me and mother). The taxes on rents receivable and deductions on housing loans were paid by my mother as income tax. Though I was listed as a co-borrower, I couldn't get any loan deductions in my ITR My sister got married in 2018. We have given another property as A Relinquishment deed of joint property to her. my BIL( brother-in-law) is a greedy person. He is demanding us to give a part of our building too. So, my mother has decided to Gift deed the property to his son i.e., me, to not have any disputes further. 1. How to execute the gift deed. Do we need to have signatures of my sister and BIL in the deed document ( as a witness) compulsorily? 2. As the property is Mortgaged under