Yes, YTA. Ian shouldn’t be sneaking out to go drinking at 15, but he was responsible enough to go to Joseph’s apartment instead of staying someplace unsafe. Ian is the one that you can be mad at, not Joseph. Joseph was nice enough to take care of your son, who was clearly less afraid of his brother than you, and made sure that you would be contacted via his girlfriend. He let Ian recover from the worst of the alcohol before sending him home, and that speaks volumes about how you parent your children.

Thank Joseph for looking out for his brother, and calmly ask him to send you a text sooner next time. Ask Ian why he snuck out, and avoid over-punishing (he will do it again).