Well in all honesty the 18 year old was very irresponsible if he had
to go somewhere he should of taken his brother with him. Now he has a
child welfare organisation on his butt, and they are people who are
ruthless and take no prisoners. Plus he could have a record. He is 18
so he is legally an adult, so not much you can do other than pay legal
bills and tell him how much hot water he is in.
Now I am wondering why you are taking the fact that your 18 year old
son left his 6 year old brother alone, so lightly. I would be telling
the 18 year old that he had better be pulling his head out of his butt
and start making better decisions and being more responsible.