in many cases tax records are useless because they don't reflect cost.
A joint tenancy is a concurrent estate in which two or more persons have an undivided interest in the same property. In California, joint tenancies must be created via the same instrument, usually by deed or by will; each owner must also receive the same or equal interest in the entire property. Joint tenants receive all the same rights. Joint tenancies often include a right of survivorship, meaning when one tenant dies the others receive the deceased owner's share.
i would take the position that since your mother made you a joint tenant you inherited her 1/2 when she died. that means you have two values you need. her cost basis for the 1/2 gifted and FMV of the other 1/2 when she died. you probably will need to contact a realtor in the area who would be best at providing estimates of the values you need. the IRS could challenge the values. they have their own experts.
