If there is nothing in the estate as you indicated, there is nothing to renounce. If you are not liable on the mortgage, then you can just inform the mortgage company of her death.
I agree with Ms. and Mr. Adams.
The legal analysis of any situation depends on a variety of factors which cannot be properly represented or accounted for in a response to an on-line question. Any answer, discussion or information is intended as general information only, is not intended to serve as legal advice or as a substitute for legal counsel, and should not be relied upon in making any decision. If you have a question about a specific factual situation, you should contact an attorney directly.