It doesn't sound like her brother is capable of abiding by a RO, or maybe even understanding what it is. What would be the point? Could he be arrested for violating it? Would that help anything? Just wondering.

Technically we don’t have restraining orders in Missouri, but we do have ex partes (a restraining order can be used against anyone, an ex parte needs an established relationship). Here are the details and requirements to get one. This should qualify under Domestic Violence and Assault. An ex parte is active as soon as it’s been filed but it needs to be served by a sheriff’s deputy (civil division usually does this) to the respondent (who you are filing it against). If he can’t be tracked down, it’s going to be hard to serve it.

And ultimately, it is just a piece of paper. While he can be arrested for violating it, it won’t prevent anything from happening, especially when he’s in a state where he’s not really in touch with reality and may not be able to register what it is and what it involves.