Don't try to be your own lawyer.
Your capital gain is calculated from her cost if the house was a simple gift, but it is calculated from the fair market value on the date of her death if it was an implied life estate (meaning she had the right to live there until she died and you had no real right to sell the property until she passed.)
It would have been much better for the deed to have been explicitly written as a life estate (we can't see the deed, so maybe it was). An implied life estate can be shown by the facts and circumstances if it was not written down, but you want to document that as much as possible in case of audit (emails, phone calls between siblings and mother expressing her intentions, etc.). You may want to speak to an attorney about proving the life estate before you file.
