I am sorry for your loss.
You need to contact an attorney in your area who can handle a probate for you. You will need the attorney to help to file the probate and name you as personal representative of the estate. Once you are appointed as personal representative, you will have the legal authority to sell the building or give it back to the bank. Since your sister did not have a will, the probate will be done under the laws of intestacy in your state. Under Section 852.01 of the Wisconsin statutes, if there is no spouse, no children and no parents, the assets pass to brothers and sisters.
If you don't want to open a probate, the attorney may have some other options for you.
This answer does not constitute legal advice and does not and is not intended to create an attorney-client relationship. The law may vary depending on the state in which you reside. It is intended only to give some direction in which to seek assistance.
Circular 230 Disclosure: Pursuant to recently-enacted U.S. Treasury Department Regulations, I am now required to advise you that, unless otherwise expressly indicated, any federal tax advice contained in this communication, including attachments and enclosures, is not intended or written to be used, and may not be used, for the purpose of (i) avoiding tax-related penalties under the Internal Revenue Code or (ii) promoting, marketing or recommending to another party any tax-related matters addressed herein.
Based upon the facts that you provide, you will need to open a probate. If it were me, I would contact a probate attorney in your area. It sounds pretty straightforward. You do not mention whether you have/had any other siblings. However, the probate attorney will certainly ask you about that when he/she is meeting with you.
You are unclear regarding the value of the property in relation to the loan amounts. If the property is worth less than what is owed, it may not make sense to list the property for sale. It may depend on a variety of other factors including what other assets your sister had at the time of her death.
Talk to an attorney!