The step up in basis (for the remaindermen) occurs on the date of passing of the last life tenant (I am assuming that your stepfather had a life estate).
Question: Was the property actually titled in you and your siblings' names in 2008 when your mom passed? Thereafter, did you and your siblings allow your stepfather to live in the house until his passing (whether as a rental or just gratis)? If so, then your basis would be the FMV in 2008 on the date your mom passed. If that's the case, you could get a licensed appraiser to do a retroactive appraisal.
