My sister passed away. Are we required to pay her bills/mortgage?

Please accept my condolences on your loss.  Your sister's estate is generally responsible for paying her bills, not her family. However, there are several cautions that accompany that general statement: 1. If your sister owned real estate (you mentioned a mortgage, so it sounds like she at least owned a residence), then someone will ideally need to open the probate estate and get the property sold or otherwise dealt with, especially if she did not have a Will. That is because there is a potential that certain kinds of liability that could arise with regard to real estate can become the personal responsibility of her heirs if her estate is not administered and the property dealt with. 2. If your sister's probate estate has any assets, those must be used to pay her expenses and debts, but only in the appropriate order, and no beneficiary or heir can receive any assets until those have all been paid in full. If debts and expenses are not paid in the proper order, then the personal representative of the estate, as well as heirs or beneficiaries who received assets, can become personally responsible to an extent, and will have to repay the incorrect distributions or payments. Therefore, who ever decides to take on the role of executor (if there is a Will) or administrator (if there is not a Will) must be VERY careful and should ideally have an attorney help. 3. If your sister has either a surviving spouse and/or surviving children, any one or more of whom are under 18, the spouse and minor children may be able to step in front of many of the creditors (but not the secured debts, like the mortgage, or federal taxes). And, if someone decides to administer the estate, then ideally someone will pay the mortgage and bills needed to preserve estate assets while the estate is being opened. Please do consult with a good probate attorney for further help in figuring out what to do.