Legal Question in Real Estate Law in California

I would advise cooperation over stonewalling. There are methods provided in the law for an unhappy co-owner of property to force its sale through a court proceeding called "partition." You want to avoid the legal costs and uncertain financial results of a forced sale if at all possible. As your question suggests, there are (at least) three out-of-court settlement possibilities: (1) she buys you out; (2) you buy her out; (3) the two of you agree to list and sell the property and split the proceeds. If it goes to a court-ordered sale, the court will take evidence on who paid the purchase price and who paid for the improvements, repairs, taxes, insurance, etc., and will try to make the fairest possible apportionment of the net proceeds of sale. Any private, out-of-court settlement should take this into consideration and do likewise to the extent the parties are able to reach agreement on things like "sweat equity." One possibility is to hire a trusted third party to act as a "rent-a-judge" and arbitrate what division of the sale proceeds is fair. Get any agreement for arbitration in writing and use someone with appropriate knowledge and trustworthiness.