I understand you want to be at peace with everyone, but you are not the one throwing a wrench into that plan.
If you mail it to his family, and they go through it, it is because he did not take responsibility for getting it from you in the first place. It is another consequence of his own choices and behaviors. Consequences are things we all, as adults, have to live with every day, and it is no one's job to manage them but our own. Managing someone else's consequences is enabling behavior, and when we engage in it, we treat them like children. That is not a foundation for any adult relationship.
He has options for avoiding conflict with his family and has elected not to use any of them. He has options for dealing with his debt and he has elected to pretend it doesn't exist. These are his problems to solve, not yours. Allow him the dignity, as an adult person, to handle his own affairs.
Meanwhile, this stuff in your possession is keeping you focused on him and his well-being instead of your own. Let it go, so you can begin to heal.