Each sibling has their own policy, or none. When an uninsured sibling breaks something, every insured sibling gets the call:
Your premium today: $30. Tom's pocket: $80.
The first insured sibling to say yes takes it. Nobody answers by the end of the day? Then it's "leave them to it". A sibling who can't pay carries a personal debt: the client waits at the door for them at the next job, and their personal buys are blocked until it's paid. The company never is. Once registered, company insurance covers everyone.