I think that this should be handled by talking to the younger brother first, since he came to the offline casino on his own will and even stole from his sister. ~
I guess a friend of this boy is involved in gambling. If one ( OP's best Girl family member) keeps an eye on him and finds out who is encouraging him to gamble. Then complain about that boy to his parents. Or, this boy (Op's best girl's brother) should not be allowed to meet him and should be explained that his friend is putting him into a bad habit.
This is more than wrong because minors should not be involved in gambling,
When I watched this video where a 13-year-old kid says he started soccer betting in 5th grade, I was shocked.
Link:
https://youtu.be/2x1hl8UE8PM