If a descendant, ancestor, or descendant of an ancestor of the decedent, fails to survive the decedent by 30 full days, he shall be considered to have predeceased the decedent for purposes of intestate succession, and is not to be entitled to the rights of an heir. If the time of death of the decedent or of the descendant, ancestor, or descendant of an ancestor of the decedent, who would otherwise be an heir, or the times of death of both, cannot be determined, so that it cannot be established that he has survived the decedent by 30 full days, that person shall not be considered to have survived for the required period.