Though without hitting the law books to see if my recollection suffers from a gap, I do not recall any case law precedent for one pilot's surviving family members suing based on a theory of failure to provide a competent second pilot. Maybe such a precedent does exist (I'm doubtful).
This is equivocal because it indulges patience of qualified posters enough to be a lawyer on this forum in the first place. So without something more definite to work from, no comment on possible legal theories for such putative claims.