This scenario is actually not at all complex; your father's wife had a life estate with you as remainderman.
As a result, you attained full ownership (in fee simple) when your father's wife passed. On that date you took the property with a basis stepped up to its fair market value on the date of death of your father's wife.
If you enter "inherited" as the date acquired, that will give you a long-term holding period which is the correct treatment for property acquired from a decedent.
