、。
If you were to die, I wonder what would become of your family.
Literal
By-chance you [subject-が] died-[conditional-たら], your family [topic-は] how become [deliberative-のだろう].
どうなる ('what becomes of') is a verb-form question, with the subject's eventual state as the unknown — distinct from どうする ('what to do'), which asks about action. のだろう adds a musing, contemplative 'I wonder' to the question — softer and more reflective than だろう alone. The 万一+~たら opening sets up the scenario as a remote, sobering hypothetical, common in conversations about contingency planning or insurance.