@baaadknight , as you will have noticed @Carl and I each assumed a different acquisition process. Champ Carl assumed that this is a conversion from home to rental and his answer is absolutely correct. My answer assumed that you two siblings have bought this property jointly and therefore answer found no place for FMV.
However there is a third acquisition path ( probably most germane to your situation) --- this was acquired by you and your sibling through inheritance.
If this is true then your joint basis is the basis of the estate i.e. with a step up/down to FMV at the time of passing of the decedent. In such a case (and assuming that the FMV is still reasonably same as at the above date, mentioned ) then FMV is the basis of the property jointly shared. Generally and absent some other arrangements, the share of each of the two siblings would be 50%.
As I mentioned earlier taxable value is not a good figure to use for allocating value to the land that property sits on ---- this is because in some states like MI, taxable value is 50% of assessed / equalized value. That is I suggested contacting the local assessor's office to see the land value they assigned to the property --- it is often 1/3 to 1/2 of the total valuation.
I hope now I have covered all the situations of acquisition ( except gift from a living donor )
pk
