If you, and your siblings, were actually placed on the deed as joint tenants with rights of survivorship (JTWROS), then as each joint tenant dies, the remaining joint tenants acquire the deceased joint tenant's interest by operation of law. As a result, you need to know the fair market value as of the date of death of each joint tenant who has passed.
As @Opus 17 stated, however, you should seek professional guidance with respect to this scenario.
