You aren’t being “stupid.” Boundaries in a family are important if people are to feel safe. You’ve learned a hard lesson about the dangers of drinking and playing a risky game. You are both old enough to know better. There may be more of an attraction between the two of you because he is not a biological brother and you didn’t spend your early years together. It may be that you haven’t gotten along because it was a way you two kept the attraction under control. However, that doesn’t make it okay. It speaks well for your character that you know it.
Sometimes adopted kids test the limits to test their adoptive family. Sometimes they aren’t 100 percent sure that they have been 100 percent included in their new family so they do something that will test the family’s commitment and to see if they will be treated like the biological children. Some families would fail the “test” and would throw your brother out of the family for not acting like a proper, protective big brother. Other families would be disappointed and angry but would see his behavior – and yours – as a reason to discipline you both and maybe get the family in family therapy. I hope your family is the second kind. Having been adopted so late (at 13), your brother may question whether he is lovable enough to really have a family. As for you, you need to be relieved of the guilt so you can have an easy relationship with your brother and so that you can feel good about yourself again.
I hope you can tell your parents what happened, not to get him or you into trouble but because there is trouble in the family that needs to be dealt with. I hope you have the kind of parents who will understand that and who will do the work necessary to make the family feel safe again for both of you.
I wish you well.