Should be said there is nothing wrong with "touching yourself." Around Abbie's developmental age it is normal to explore those sensations. It is to be expected. Even "humping inanimate objects." Sadly Abbie will never grow out of that either. But it isn't a shameful thing.
But her parents think that it is fine to let her to whatever, whenever. Ass seems to enjoy watching her do it as often as he films her "stimming." They say how free Abbie is and isn't worried about society pressures but that isn't a choice she makes. And even if they say they don't care if it makes others uncomfortable they SHOULD care about maintaining Abs dignity and safety.
Teach her safe and private ways she can be "free." And by private I don't mean where her dad or brother are participating or watching. (I would include P in that but let's be real she wants nothing to do with Abbie if it doesn't directly benefit her.)
It is possible they are just lazy and don't care.