Who should have gotten the seat belt ticket?

my son is 25 years old and mentally handicap when he is in the car with me i make sure he buckles up well he was with his aunt my sister-in-law they got stopped he got a seat belt ticket i didn't know it until a police Officer came to arrest my son for failing to pay the ticket i lied and said he was not there he would have freaked if that happened i found the ticket in his coat he had no idea what it was for i took him and went to courthouse to pay for it and i told them they should have given the ticket to my sister-in-law they told me since he was not a child he would get it i think that if they are mentally handicap the driver should get the ticket