The primary issue here is that your parents seemingly reserved a life estate for themselves.
As a result, you inherited nothing with respect to the home because the life estate would not terminate until the last of the life tenants have passed away. Since your mother was still living at the time of the sale, she would be entitled to precisely what she received for the value of the life estate she relinquished while you and your siblings would be entitled to the value of the remainer (future interest).
Therefore, you do not need to conduct any kind of analysis into the fair market value on the date of your father's passing since the only thing that happened related to the home would be your mother became the sole life tenant.
