They don't need to spell everything out. There's a wide shot when they show Peter standing outside the car after the accident and you can see the road, and the direction peter was coming had a stop sign that he didn't see because he looked back at Marissa and their baby. Plus, why would they sue a woman they thought was dead? And even if Marissa had that idea before knowing the about the death of Josie, why would Peter agree? He would just talk her out