What are your plans for the house? If it will be vacant, it may be impossible to insure. You will need to hire an attorney to evict her, as you cannot represent your father in court without being an attorney. You will also want to consider whether this is what your father would want, and what long term effects it will have on your relationship with your sister. It can certainly be done. I would consult with a landlord tenant attorney.
*** LEGAL DISCLAIMER 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 your state. The law changes frequently and varies from state to state. If I refer to your state's laws, you should not rely on what I say; I just did a quick Internet search and found something that looked relevant that I hoped you would find helpful. You should verify and confirm any information provided with an attorney licensed in your state.
I agree with the other attorney. You need to talk to an attorney immediately. You cannot afford NOT to talk to an attorney.
Since your step-sister is living in the house the only way to get her out of the house is to have a judge evict her.
In the meantime, if the utilities are in your father's name, you can have them turned off. Of course, she might "trash" the home if you have the utilities cut off.
I would also talk to your insurance agent regarding the potential problems of having a vacant house. Also, regarding what happens if she damages the house. You want to make sure that the insurance is current on the house in case it burns down.
I would also talk to your local police, sheriff and/or constable about your options. Since this is her current home, I believe that she can keep you from entering the house. I would call and give law enforcement a heads up about your family's situation - after consulting with an attorney.
You have a lot to think about. This is not something you should try to resolve on your own. You should consult with a local probate attorney for a thorough review of the circumstances. As for getting your sister out, you can seek to have her evicted, but you need to consider what your father would want and what your sister might do. As his agent, you have to act in his best interest.
Did your father have an estate plan?
When responding to questions posted on Avvo, I provide a general purpose response based on California law as I am licensed in California. In reviewing my response, you are specifically advised that your use of, or reliance upon any response I provide is not advisable. I do not have all relevant background details or facts related to your issue / matter, thus I am not in a position to give you legal advice. Further, your review, use of, or reliance upon my response does not establish an attorney-client relationship between us nor does it qualify as a legal consultation for any purpose. For specific advice regarding your particular circumstances, you should consult and retain local counsel.