You can't find a way to resolve the problem under Immigration Ac

Usually if the principal applicant died beforeI-485 approval,his or her family members can't get green card because the base haven't existed.Unless in very
special stuation.I remembered one case such as:a chinese women who was a U.S citizen,she died before
her mother's green card be approved ,USCIS intent to
deport her even didn't care that 2 kids needed her to
care them.Finally two Congressman interfered the case & help her getting green card.I think that is only
way if they want to live in USA in future.