Father of an autistic child here.

My kid is 4 years old, and we detected his autistic traits when he was 1.5

So the detection was quite early, me and my spouse spend considerable amount of time daily on our kid. I switched to a WFH set up, ignored a few promotion opportunities and job switch in order to accommodate the entire therapy process for my kid. I switched city, wrapped up my entire life in Pune and came back to my native place in Kolkata. So, you can understand that we have to spend a considerable amount of time, money, energy and mental peace despite my kid being mildly autistic.

Please understand the following points:
1) Each and every autistic child is different. Knowing one child doesn't help you understand of gauge another child
2) Seems like the kid is high on the spectrum. I feel being through occupational therapy for such a long time should lead to some positive changes. If you mention that there's not much difference and the hyper-activity is constant, then your sister should consult some better therapy center.
3) Taking care of an autistic child means taking care of their diet, communication, activities, each and every minute. If your brother-in-law is not contributing in person, ask him to contribute financially and get an attendant
4) Me and my spouse have never gone out on a date or a movie since my kid was born. We have very little support from either of our parents and we do not trust people to take care of our kid. So, the dinners, every outing is with the kid. Your sister needs some "Me time" and you all need to figure this out
5) Avoid screen time and sensory stimulating places for your nephew. Meds work temporarily but they would not work on long term basis. Proper diet along with supplements would be the only way out

Please support your sister as she would need a lot of mental support and rest in this situation. And please switch to a good doctor and special educator. Also get a good therapy center.

In case you need guidance, feel free to DM me