Re: tenants in common
Who set this deal up in the first place? Sounds like a patchwork quilt. Is there any kind of written agreement? Which of them made the down payment? What was the purpose for the combined ownership and what was the original plan or understanding about what would happen down the road? All of these questions have a potential impact on the outcome in court, if it has to go that far.
The items you mention such as payment of expenses do have an impact on how a court would settle this out, but only as items of reimbursement rather than affecting ownership percentages.
The basic legal tool to un-do unhappy cotenancies including tenancies in common and joint tenancies is a specialized kind of lawsuit called a partition action. The co-owners should try to resolve their differences out of court, of course, and the process should include each party getting legal advice as to what would happen (and what wouldn't) if the dispute went to trial. More often than not, when both owners really understand the law, the dispute can be reconciled through a written agreement (which should have been written when the cotenancy was set up in the first place), or by a voluntary buy-out.
If a partition suit is necessary, the parties may still settle voluntarily before trial and a final court ruling, of course; the judge and the court system will put pressure on the parties to settle before trial through mediation or some other process.
I don't understand the reason given for the father-in-law's unwillingness to cooperate, or what influence the sister/executor has here. The role of executor should be unimportant if the testator is still living; executors can be changed easily and perhaps should be here.
Giving you good, useful advice would require more facts and some numbers, but getting the best possible outcome promptly and at low cost seems fairly easy; the parties will probably come togther when both fully understand the law. One of the biggest detriments to negotiated settlements of property disputes is that one party or the other has misconceptions about rights, obligations, valuations and interests.
I do about 75% of my work in Sonoma County and real property disputes of this kind have become a sort of specialty for me recently. Call or e-mail me for a free consultation if you wish.