Cosigning is often just for the debt and not for ownership. We don't know the situation here. Anyone can sue anyone for anything. That sounds like it may be a simple negligence claim which isn't based on any actual ownership of the vehicle. We can't know for certain.
Liability is a fact-specific issue.
A resolution depends on all the facts surrounding the issue. We can't possibly know all the facts. If you are being sued, your best bet is to get to an attorney. If you have any insurance which may cover this sort of thing, you likley should put them on notice. Ask an expert or become an expert .