My wife grandparents live in Adelaide Australia and recently…

My wife grandparents live in Adelaide Australia and recently her grandmother has passed away. We live in Perth Australia but their step daughter whose connection to my wifes grandparents was through marriage to their son who died 14 years ago live in Adelaide also. Both of my wife grandparents are in the nineties and over the past 5 years it seems as though this step daughter has coaxed these people into passing over a lot of their belongings which were originally destined for my wife. We understand she lives there and would obviously be involved in their lives constantly but on the passing of her grandfather (if and when this happens) my wife has a suspicion that the will has been set up to benifit the step daughter. My wife is the only blood relative alive in the family with her mother dying when she was 6 years old obviously has taken a very strong interest in her grandparents growing up. My wife is now 40. What avenues are available to ensure a fair settlement. We have looked online a few times and have noticed that being a grandchild seems to bear no real significance, only children etc seems to be entitled to challenge a will. Hopefully we are mistaken but are eager to know what avenues are available when the enevitable happens. Regards, Chris.