OK, so the family was out to dinner, and my SIL left early, leaving my MIL in charge of her son (6 y.o). MIL has car seats. However, she forgot they were in FILs truck and she had no car seat. I told her she needed to call SIL and have her come back, but she chose to let him sit in the middle seat with no car seat, and then she did not tell SIL that he had ridden home with no seat. When she found out, she was not happy, but chose not to confront her mother.
So when we got home, I talked with DS about how he could never ride in a car without a car seat, and he told me that G-ma had let him & his cousin ride in the back of her car with just seatbelts at the lake. I re-iterated that this was not OK, and if anyone ever asked him to ride in a car without a car seat, he needed to get permission from mommy or daddy first. DH talked with her and explained he did not want Alex in a car w/o a car seat. But she seems more concerned with convenience that safety.
Any suggestions? He loves visiting Grandma, but this is not the first time we have clashed on car seats.