Very sorry to hear of your loss. You'll need to report this on your Dad's final return, it's considered to be a "deemed disposition".
When completing your father's return, in the Profile section of his return, you'll need to answer "Yes" to the question "did you dispose of a property in 2018 that was your principal residence". Then as you continue through your EasyStep interview, you will come to the section "Sale of your Principal Residence". Here you will enter the details of your father's share of the property, and even be asked if it was disposed of as a result of his death.
