In both Missouri and Illinois, there are strict rules on who can file a wrongful death lawsuit. In Missouri, there are three classes of eligible individuals. Class 1 includes parents, spouses and children. If there is no one in this class, then individuals in Class 2 are eligible. Class 2 includes siblings and descendants. If there is no one in Class 2, then the court can appoint a Plaintiff Ad Litem to prosecute the case.
Illinois has a similar system. Essentially, the next of kin is eligible to file a wrongful death case. In the first instance, this includes spouses and children. If there are none, then parents can file, then followed by siblings.