1. This may not be a good idea of getting the property transferred to your name at a later date while you pay the price now.
What if your sisters demand more money for the same property at a later date when the proposed Will becomes enforceable?
If the property belongs to your parents then the question of settling any amount to your sisters during the lifetime of your parents with regard to this property do not arise as neither they nor you have any rights in their property at this point of time, hence when the Will becomes enforceable your sister, on the ill advise of some mischief mongers around, may claim a share in the proeprty or demand more money, if refused they may not sign NOC or they may even file a partition suit, at that time this settlement amount may not be considered as a legally valid item.
It is better you ask your parents to execute a registered Will in your favor now itself and can recite in it that you shall pay them the amount what you have proposed to give them now at the time of enforcement of this Will as their share in lieu of a share in the property.
This arrangement may be a safer mode for now instead of worrying about the possible litigation in future in this regard besides settling an amount now itself to them.
Alternately you can ask your parents to execute a registered sale deed in your favor for the proposed amount now itself, and lt them keep the money with them till their lifetime which can be bequeathed equally to all their children by a registered Will after their lifetime, so that the property is also safe and your share of money is also safe.
Discuss with an advocate in the local and decide further course of legal action in this regard.