You are playing inside a game which you didn't create, and don't set the rules, so don't have any obligation to anyone except yourself (and your brother). You spoke of an exit strategy, which is GREAT, since you don't owe anyone an explanation, even your adopted parents (who likely are NOT going to change, or even support your decision to leave).
The key is to "stay the course", keep your cards close to your chest, and take the steps needed to ensure YOUR ability to leave the organization in the future (college). Acting slightly nutty is being smart like a fox (in my book), so do whatever you need to do without sacrificing your own morality.
Like I alluded to, in the final analysis people can only control THEIR actions, take control of their OWN lives, and no one elses. Sad, but it's the truth.
There's a great big beautiful World out there, and don't let anyone dampen your experiencing it to the fullest of your capabilities! It may not lead you where you'd hope or think, but that's sort of the beauty of life....