The answer depends on how the will or trust of your grandparents was phrased. Based on the facts you describe, the normal course would be that you and your siblings would receive your father's share. But that is not always the case. One needs to review the estate planning documents.
Your question is going to be governed by whether there was a trust or will. If there was one of those documents then it will provide how the assets of your grandparents' estate were to be distributed. Assuming there was a trust or will and your father's share of the estate passes to you upon his death, the trustee (for a trust) or executor (for a will) would be the person you should talk to first. If he or she is uncooperative then it is time to talk with an estate or probate attorney.
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If your grandparents did without a will or trust which controlled what happened to the house, then you and your sister would be entitled to your father's share. However, as stated above, this could change if there is will or trust that states otherwise. The default position would be that you and your sister get that share absent any other provisions.
You asked a very similar question, elsewhere. You sister has an equal right to your father's share as you do. Your aunt cannot simply give her your father's share. Having said that, if your aunt is uncooperative, then you are going to need to hire an attorney to make sure your rights are protected.
I am licensed to practice law in the State of Michigan and have offices in Wayne and Ingham Counties. My practice is focused in the areas of estate planning and probate administration. I am ethically required to state that the above answer does not create an attorney/client relationship. These responses should be considered general legal education and are intended to provide general information about the question asked. Frequently, the question does not include important facts that, if known, could significantly change the answer. Information provided on this site should not be used as a substitute for competent legal advice from a licensed attorney that practices in the subject area in your state. The law changes frequently and varies from state to state.