Interesting question. I assume your father gifted the property to you in June 2003 subject to his retention of a life estate. At the time he valued the gift of the remainder interest based upon the then fair market value of the entire property less the actuarial value of the life estate he retained. But if I understand you correctly, you now intend to sell the remainder interest.
Since you are selling and not gifting, you don't necessarily have to rely upon the Sec. 7520 tables. However, if you want to sell for a fair price and want to avoid any suggestion of a partial gift to your sisters, you should calculate the current value of the remainder interest. I would start with another valuation of the whole property (appraisal or realtor CMA letter) and back out the current value of your dad's retained life interested, calculated using his current age and the Sec. 7520 tables.