No. This is not permissible and seeing her grandparents is not a legitimate reason to travel without a mahram. It is not permissible for her to travel to perform one of the pillars of Islam without a mahram.

Is it permissible for a sister to travel who has no mahram to travel by herself, on the condition that her grandparents are in their old age and death can happen any minute and the sister wants to see them?… …Also she has to wait 3 years before she gets married and in those years time the grandparents may pass away. On this condition is it permissible for her to travel without no mahram?