Exactly where did sis get her law degree from? She is mistaken.
This is just a minor mistake on your part. There was no intention to defraud on your part. The executor of the estate needs to be talking to the mortgage company to determine if it is necessary for the house to be sold or not, or if you want to continue living there, consult with the executor and inform him/her that you are willing to refinance the mortgage with YOUR name on it.