I would recommend getting a CSF study done.

Chiari's is defined by the Cerebellar tonsils descending into the foramen magnum by however many mm... but the symptoms seem to depend more on the circulation of the Cerebral Spinal fluid than the actual tonsils, themselves. Some people have a 1mm escopia with many symptoms, and there are others with 7-9mm escopia with light symptoms.

*edit* Hope this helps.

Good luck to your family. This is a hard thing to have to hear, but keep your chin up, stay strong, and you will get through it.