Can a co-signer repossess a vehicle if a sibling is not making payments?

The co-signer is typically a person who signs for the car loan; the co-signer may or may not be listed as a co-owner on the title.

If the co-signer is also a co-owner, the co-owner who is paying the loan can legally take the car. However, if the co-signer is not a co-owner, taking the car would be grand theft auto.

Incidentally, if the co-signer is not also a co-owner and the loan falls into default, both the signer and the co-signer will be identified in credit reports as defaulting on a car loan, even though the co-signer did not have or use the car.