Towing Issues: towing problems, sister and brother, brother and sister

Question
My father has a very difficult situation and I would greatly appreciate any input. My father helped his sister and brother to buy three cars and neither of my father's brother or sister has a good credit so my father to help them put the cars under his name(credit) when the cars were bought. His brother and sister were suppose to pay for the car each month, but since my dad is the owner they decided not to, so my father has had to pay for all three cars each month or his credit will be ruined. However, we are tired of paying their bills and we want to take those cars away from them. How can we take (tow) away the cars from them withougt doing anything ilegal. We want to return the cars to the dealers and just get rid of the cars. What can we do?

Answer
while it does sound kind of easy to answer this question, its kind of hard.

If the titles are in your fathers name either as owner or lein holder, he can have them taken back to the dealer if he so desires.  The problem is that some people will fight any and every thing that is done. It will be hard to do what is right for the family.  Sorry that some of them just do not want to do what they agreed to.