Additionally, your basis in the property is likely the fair market value on the date of your parents' death (specifically, the last to die). Ignore the assessed valuation since it is not relevant for the purposes of determining gain or loss on the sale.
You need to check with your attorney to ensure that adding your siblings to the title was not a gift (and it does not appear as if that was the case). It is more likely that you and your siblings acquired a remainder interest and your parents reserved a life estate (in which case the FMV on the date of death rule would apply).
Again, check with your attorney.
