In my opinion, since ther have this only daughter as ther successor in interest, moreover the properties in their native are under the care of their close relatives, it would be in the interest of safeguarding the daughter's interest i the property, the parents may execute a registered conditional settlement in favor their daughter now itself. In the conditional settlement deed, it may recited that the parents have lifetime interest in the property and the she will be able to acquired the properties only after their lifetime.
The Will whether registered or not, it may be challenged by the relatives at a later stage by creating a fabricated and fake document on a date prior to the will and she may be put in trouble.
If it is a registered settlement deed, the properties can be transferred on her name now itself with the concerned authorities during their lifetime and any opposition by the relatives to this may be done now itself so that the future problems can be avoided. In this arrangement also the lifetime interest will vest on the parents only.
Consult a different advocate on this and proceed after analysing all the facts and future problems