You need to either go through the judicial process or offer her some $$ to move out.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here. Please visit my web site: www.avanesianlaw.com for more information about my services.
Redeeming the mortgage does not automatically transfer ownership.
The surviving spouse may have some vested interest as a spouse.
You will need an attorney to represent you or your sister to deal with the spouse
and understand the rights of all the parties.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.