anti-psychotics are not known as a drug that can give relief to depersonalization disorder in its primary form. The depersonalization research unit wrote in a publication that in over 500. patients they had seen some saw their depersonalization gets much worse if a psychiatrist mistook the disorder as a psychotic state and they where given anti-psychotics in high doses. So, few might see some benefits in low doses.
Depersonalization is both a disorder and a symptom. It can be a symptom secondary to many psychiatric and organic states. In psychiatric is in seen as secondary to states like anxiety, depression, OCD, borderline disorder. When it is secondary to something it should in many cases go away when the primary cause is addressed. A low dose of anti-psychotics can work on anxiety. I think you mother have a anxiety condition and with secondary symptoms of depersonalization and might explain the response. You could try some anti-depressants or a drug like buspirone for her anxiety. Depersonalization symptoms will follow out with the anxiety.