Hello Monaliza,
Do hope your sister is progressing well with her recovery.You are naturally so concerned for her welfare.
Am posting a non expert reply for you here, and would certainly think that an early psych consult would be helpful for the longhaul. My thoughts come from the viewpoint of a retired doc (not psych), but just might reassure you somewhat re the lack of regular antipsychotics at present...
My understanding would be that the hypoxic encephalopathy would have caused variable physical damage and injury to areas of your sister's brain, often particularly the basal ganglia and brain stem. A healing process is taking place but for the moment those injured areas will be very VERY sensitive to psychotrophic drugs, and their side-effects. The balance of different neurotransmitter systems is likely to be unstable and could shift rapidly switching 'off' or 'on' of particular pathways so long acting meds will be used cautiously.This is very similar to treating the very elderly or after head injury...So I would think an 'as required' medication plan would make sense.
It is good that you are there as her advocate and will be alert to any distress and report it early to the medical team.
Hope this helps a little,
Best wishes,
Chris, UK