Demand of 50% share on a land property previously settled and agreed

I am the eldest son to my mother & father both are alive and living with me. Next to me is a sister and the last one is my brother. About 5 years back after the demise of my grand mother(mother side) my mother become the owner of her father's(died about 20 years ago) land property of 2400 Sq.Ft . My mother also owned a separate land property of 1800 Sq.Ft. My mother gave her father's land to by brother and her land to me about 5 years back. The disbursement of these properties for me and my brother are documented and registered as well. My sister has given other benefits by my mother. At that time since my brother was in good book of mine, I did not bothered about the difference in land area and the value of said property given to me by my mother and I accepted the same. But now the relationship is strained between me and by brother since he was trying upper handedness over me and I positively decided parting away from him.Please advice me how I can get 50% share of 2400 Sq.Ft land .