Did she retain a life estate in the deed to them? That is a contract and is probably recorded at the courthouse. As I understand life estates, she could only sell what she has....a life estate, that is the right to live there until she dies. All she could sell would be her interest meaning that any purchaser would be acquiring an interest for the rest of your mom's life. When your mom dies then the house would revert to your sisters and they could do with it whatever they desire. Any buyer should beware of what they are buying.
You need to check on the state version of the Fraudulent Conveyance statutes. Love is not a valid consideration for a conyenance to be protected from a creditor who attacks the validity of the conveyance.