The basis as to the remaindermen would be the fair market value on the date of the last life tenant to die.
As to the descendants, they take their share through the those who were granted a share of the fee after the death of the last life tenant and, as a result, their bases (plural) will vary depending on the fair market values on the death of each remainderman.
This is rather complex and you should seek the guidance from a local tax professional and/or legal counsel.
See https://taxexperts.naea.org/listing/service/estates-gifts-trusts
See also https://www.avvo.com/probate-lawyer.html
