Legal Question in Real Estate Law in California

No. In fact as co-owners you owe zero rent except by mutual agreement. Each co-owner has a right to use and occupy the co-owned property at no cost. Usually, when one lives in a property and the other doesn't, they agree on compensation, such as you two have. But that is a contract, and contracts cannot be unilaterally modified. So she had no right just to change the amount by giving notice. There must be mutual agreement and separate consideration. In addition, a verbal agreement regarding use of real estate is only good for up to one year. Longer than that and it is unenforceable. So if you have not agreed on a payment in the last year, you technically owe nothing.

It sounds as though you are a co-owner. You might want to verify this by a records check. If you are indeed a co-owner, you can live in the house without paying rent to the other co-owner, unless and to the extent you have agreed to pay. All co-owners of real estate have the right to occupy the property without payment of rent to the other co-owners -- unless they have contracted to pay something. The enforceability of your oral agreements to pay rent is questionable after the passage of time. I'd recommend having an initial free conference with a local real-estate lawyer to verify whether and to what extent your oral agreements can be enforced against you and to get verification of what I say about your right to occupy the property without paying rent.