@user38615 If the total sale price is under $250,000, then you do not have a taxable gain since you owned the home and lived in it for more than 2 years prior to selling, and no matter what the basis was, your gain must be less than $250,000.
If the sales price was more than $250,000, then we need more information to determine your gain.
Specifically: 1. exactly what was the form of the deed when you were added as a homeowner. Was it "in fee simple", or did your mother retain a "life estate"? You may need to look at the transfer paperwork from 1999, which will be on file with your county. "Life estate" has many similarities to "joint tenant with right of survivorship" but may not be identical. You may need an opinion from a real estate attorney in your state.
If your were "joint tenants" then each of you had the right to sell their half of the property without the permission of the other; you were an equal co-owner. However, if your mother had a "life estate" that means that you could not sell your half without her permission.
If your mother had a "life estate" then you inherit a fully stepped up basis to the day she died in 2016.
2. if your mother did not have a life estate, then to figure out your basis, we need to know when your father died, since they owned the home together. Was it before or after you were added to the deed? And when was it?
