Ask for the key in writing, document the refusal and then you can file an action to partition the home.
You should consult with an attorney to do this as it is not trivial to do it correctly. This is a real estate issue, not a probate issue.
My comments are not intended to establish an attorney-client relationship, are not confidential, and are not intended to constitute legal advice. Proper legal advice can only be given by an attorney who agrees to represent you, who reviews the facts of your specific case, who does not have a conflict of interest preventing the representation, and who is licensed as an attorney in the state where the law applies.
The only real action you have against your sister is an action in partition. I agree that asking for a key and having her deny it would help a case against her for pre-partition damages although it is very hard to prove in these types of situations. Since it is a home an is not actually suitable to cut in half the judge in a partition action will most likely either direct that you all sell the home through a realtor or the home will be sold on the courthouse steps to the highest bidder. We always suggest that to keep someone from getting the deal of the century you should make sure you have the means to buy the home if a courthouse sale is mandated. The personal property probably should have been disbursed in a probate matter by the personal representative. For either issue you need to speak with an attorney to resolve this issue.
This answer does not constitute legal advice and does not create an attorney client relationship. The answer is for information purposes only and is based on the limited information you provided.
Is the property already titled in your and your sister's names, ie has probate occurred? If not, that step will need to be taken as well.
I agree with the other attorneys, you will need to do a partition action.
I would not recommend you trying to do this on your own. You should consult with and hire a real estate attorney to assist you with this matter. If the property is still titled in your grandparents' names, you will also need an attorney experienced in probate administration. There are firms in the Pensacola area that handle both real estate and probate matters and should be able to assist you with your situation.