Under California (where I practice), it is not so clear that the house "doesn't legally belong" to your mother-in-law, based on the facts you provided.  You need to consult a lawyer, let the lawyer review all documents and discuss this with you further to get all relevant facts.  If it turns out that your MIL doesn't have a legal claim to the property, your primary recourse is to maintain a "quiet title" action.  We can help.  Call for a free consultation at 800-553-7626 if the property is, or your inalaws are, located in California.