Very well could be a side effect. Has your relative seen a neurologist?
My first thought was tardive dyskinesia. Only because I've seen such an illness discussed before, due to being a neurology patient, and years of reading, learning, et al. ((what happens when an MS patient, lands in PC, ok?))
I'm certainly just a patient. Not a doctor, so seeing a doctor, that can identify and figure out what is going on, is imperative

Tardive dyskinesia: MedlinePlus Medical Encyclopedia

Coping with Atypical Antipsychotic Side Effects | Psych Central