This is truly complicated. You did not say what is in the prenup. If it says you have nothing to do with the house then you can either cancel it or else you have nothing to do with the house since it is your hu*****and's separate property.
Now since you are married after the title to the house was bought, it remains your hu*****and's separate property. Your marriage does not change its character without a proper transmutation. However, you may have invested money in the mortgage with wages or other community property, such that you acquired enough community property interests that cannot be ignored. In such a case, a California court will use Moore-Marsden calculation to calculate the portion of the commmunity property interest and you are entitled to half upon death or divorce.
Now the complication is the joint tennacy. I could not find any case directly on point. Based on my understanding, your hu*****and owns half of the property as a joint tenant with his parents, or maybe one third, depending on how the title was written. Now you acquired some community property interest. Upon his death, your community property interest will take precedent and what is left is the joint tenancy interest. This is the only fair and reasonable way.
He can change this by rewriting the title to a tenant in common or even get rid of his parents from the title and give the same interest to them by a will.
