According to your question, it is your grandmother who has the right to inherit. You and your sister have the first right of inheritance and should share it equally.
But the reality is:
Your sister is too young. If she has a lot of property, she can ask for less. If she has less property, she can ask for more as her alimony.
Your grandmother is too old for the same reason as her alimony.
You are an adult, relatively few.