You may want to run this past an accountant who is familiar with your state laws and show them a copy of the deed so they can see the exact wording.

In most cases, if you held the property jointly in fee simple, then you inherited half a stepped-up basis when your parent died.  For example, suppose you purchased the property in 1992 for $80,000.  The fair market value on the day she died was $140,000 and the FMV now is $150,000.  Your cost basis is half the purchase price ($40,000) plus half the value on the day she died ($70,000), equals $110,000.  When you place the property in service as a rental, your basis for depreciation is either the current FMV or your cost basis, whichever is lower.  Don't forget that land doesn't depreciate, so you have to back out the FMV of the land.  A real estate appraiser can give you a retroactive value of the property in 2017 based on historical records.

Two other points.  You can increase your basis by the cost of any permanent improvements that were made since you bought the home, such as a new roof, furnace, etc.  (An improvement raises the value of the home or extends the life of the home or one of it's systems.  You don't adjust basis for repairs.)  Because you inherited a stepped up basis on half the house, that folds in all those improvements.  So you only adjust your basis now by half the cost of improvements -- the half that applies to your half-share of ownership that did not get an adjustment when your mother died.  In the above example, if you installed a new furnace in 2000 for $6000, you can increase your basis by a further $3000.

Second, keep all documents relating to basis and depreciation, including sales records and an appraisal retroactive to the date she died, for as long as you own the property plus 6 years after you sell.