There is probably a lot more sexual assault going on in that family than what you know here. It is very likely that A himself was a victim of SA/rape and that is how he learned these behaviours. It may even be ongoing. It would explain why they are abusive to A but then cover for his SAs: they are protecting someone else.
This is not to be an apologist for his behaviour, but to highlight new avenues to approach stopping this.
A is non-verbal, so there is a high chance he has a social worker, or at least their case might be taken more seriously by social workers. You can call social services and tell them that you think A is being abused due to behaviours he is exhibiting.