Once the NOC has been given and the property has been transfered to your name it cannot be revoked. If your sisters have not given the NOC under pressure, misrepresentation, undue influence etc. then it cannot be challenged. Be rest assured that the property will belongs only to you after the transfer has taken place.
If your sister has given you NOC then get it registered in your area and if she is having any child who are above to 18 and 21 then take their signature on the same NOC and if the childrens are minor then take your sister undertaking that the property has been given to you under all circumstances and no one from her heirs will claim in the property and if any thing happens then she will take care of this and she will b responsible for this and she will support you in future too. Better you personally meet lawyer he will do the needful.
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