Well, I would never recommend taking legal action against a family member. Maybe I'm a bit too old world and Sicilian, for that.

But, at 23, unless one is mentally incapacitated, one must take responsibility for one's actions. it may be too late to get back the items, based
on them being stolen or sold by someone not mentally competent. But absolutely, the guilty party should make restitution. Maybe a visit to the
8th circle of Hell would help everyone understand what happens when you backstab your own blood.

ANyways, i hope that somehow you find a way to resolve this. I've lost a number of precious things in life, to the point where material things
no longer hold the allure they once had. But it still hurts to lose things that have meaning.