ideas on the subject of sharing a car with a sibling.
They would need to cooperate with each other -- which could be hard for some siblings.
They would need to coordinate trips where they could ride together. They'd have to compromise if each of them wanted the car for a trip they wanted to take alone (or for a date).
They'd need to work out how they were going to divide up expenses like insurance, gas, and routine maintenance.
What would happen if one of them got into an accident and had to have the car repaired?