@Stian I never liked that and never used it. Because in almost all the cases, it really isn't complicated. It just doesn't fit the traditional relationship model and therefore doesn't have a common term.

@Tinkeringbell I'm trying to do that as much as possible. It's not always possible, but yes, where I can I've forced Nathan into suffering the consequences of his actions. It's just not always possible.

@DaveG from the lack of answers, I fear that to be the case. But no, I can't. His actions are causing problems for all around him and it has already taken too much energy and money from us to compensate for it. If he were just a friend, I'd simply cut off contact and let him be. But family is a different matter.

@Tinkeringbell to my knowledge, Nathan has never had to bear the full burden of consequences for his actions, he always managed to find someone to help him out or to shift the blame elsewhere. But that's only AFAIK.

This was my first thought. If ownership of the items in question isn't in doubt, refusal to hand them over when asked is something the police will act upon, and depending on the details and jurisdiction might well be a crime. If he refused in writing (mail, text, etc.) taking that to the police and asking that they escort her over when she collects her things should be very doable.