Relinquishment deed

Dear sir, My father and mother own a house on a 400 sq yard plot jointly. We are 4 children(first 2 from the 1st wife and the other 2 from the 2nd wife). My father s 50% is to be divided between the first 2 children and my mother s 50% share to be divided between the youngeR 2 children, thus giving all 4 children an equal 25% .My mother has made a will stating her 50% share will be divided between us 2 younger daughters, whereas my father who is 80 years old and is keeping ill health nowadays has NOT made a will yet and has told us VERBALLY in case of his demise , we should give what comes to us(mother and younger 2 daughters) from his share to the first 2 children. As in, in the absense of a will of my father's, his 50% share will automatically get divided between mother, and 4 of us children. Then mother and us two younger children should give our share from father's 50%(10% PLUS 10% PLUS 10%) to the elder 2 children. So that the property will be equally divided between all 4 children. My mother , sister and i are absolutely fine with that. however, i was wondering how this can be done!! I was reading about a relinquishment deed. So if my mother sister and i sign a relinquishment deed of the one third share that comes from father s 50% , does it mean that we have relinquished our right to the WHOLE property totally AND FOR GOOD?? As we dont intend to do that.My mother, sister and i would only want to give up or relinquish the property share of the 50% that might come to us from father s side AND KEEP OUR 50%(mother's which will be willed to us 2 later) OF THE PLOT. SO, IS IT POSSBILE TO SIGN A RELINQUISHMENT DEED GIVING UP ONLY PART OWNERSHIP AND KEEPING PART OWNERSHIP WITH US ? IF NOT, HOW TO GO ABOUT IT? Pl advise