Your depreciation will NOT mirror her depreciation.
Here is a simplified example:
She bought the property for $100,000. Value of land was $20,000. Over the years, she claimed $30,000 of depreciation. Her Basis is now $70,000 ($20,000 for the land, and $50,000 for the building). Her "placed in service" date is the date she made the building available to rent.
She then gave you a gift of 50% of the rental property. Your Basis for depreciation would likely be $35,000 ($10,000 for the land, and $25,000 for the building). You use the date you received the gift as the "placed in service" date.
As a side note, your mom is likely required to file a Gift Tax return for her gift to you.
