1. How did you get the ownership of the flat which you have gifted to your parents? Did you purchase it out of your own funds? Which right did you wish to transfer to your parents? If your parents were to will this flat to your child then you should not have executed a gift deed in their favour in the first place. There were other options available under the law to transfer the right which you wanted to confer on them. So to this extent the execution of gift deed was not a prudent move.
2. The ownership of the flat passed to your parents after you executed the gift deed in their favour. As the joint owners of the flat they are now at liberty to bequeath the property to any one. So your parents can make a will in favour of your son.
3. Your sisters can challenge the will in a court of law on the ground that the said will is vitiated by coercion i.e the the will was not made by your parents with their own free volition.
4. Since your parents are now the owners of this flat they can give the entire property to your sisters by making a will in their favour, which can be challenged by you on the ground enumerated in the preceding para.
5. To preempt an legal action by your sisters get the will drafted by a lawyer. A meticulously drafted will will sustain even the most tempestuous legal challenge in the court.