Yikes. You are in kind of a difficult situation. Even with landlords the solution is usually to change the locks and move on if a now ex-tenant doesn't give the keys back. I doubt you had a roommate agreement that set out the responsibilities upon one of you moving out. Police probably aren't going to go after a family member for the keys though I guess you could go into your local station and ask, as much as you don't want them involved.

You can see if you can find a local lawyer that does a free consultation for options. I'm not really knowledgeable to know NS's specific rules, but you could look into whether his property in the apartment is considered abandoned. If it is, that at the least might get them motivated to pick up their stuff and return the key. There are usually strict rules though about the process and options so you'd have to speak to someone better equipped. Maybe someone else will be able to chime in with more info for you.