It's hard to say from the little info you've given us, but, I would say, as an autistic person and one who's raised an autistic son with cognitive impairment, that trying to engage him by asking him questions might be a way forward.
I assume he is often playing video games? My son is very addicted to these.
It's often tricky on the phone for autistic people, but giving him plenty of airspace to put forward his side of the story will not only help to engage him away from other distractions but also give you more information with how to proceed with feedback.
Being reasonable and logical often helps, as autistic people often respond well to this approach. Being overly emotionally reactive does not.
He may struggle with impulse control and need a lot of reminders about what is fair and reasonable, in terms of expectations, but any sign of anger, disapproval or judgey harshness will likely cause him to dissociate and feel overwhelmed and may cause a shut down.
He, no doubt, will have his reasons for his behaviour and more likely than not, he is feeling disempowered and unable to verbally defend himself in response to perceived threats that cause him to lash out at his sister.
Siblings can be quite bully-y with autistic siblings and take advantage of their lack of social know-how and ability to process and respond in a timely fashion.
That is my experience, anyway.
My ASD3 son (as opposed to my other autistic children with higher intellectual capacity) is my second born of 7 children and I witnessed SO MUCH bullying and contempt leveled at him when he was growing up (he's 30 now and living in very kind, caring, supported accommodation).
Granted he was a handful, and strong boundaries and much patience and constant prompting have been needed.
Doing as much research about autism as you have time and inclination could be helpful for you. I recommend a book called "The Loving Push" if gaming addiction is an issue.
Restrictive practise causes a lot of back lashes and is further disempowering for autistic people but finding support in the community and offering different and fun activities can be fruitful and beneficial. Other than that, if life is feeling overwhelming for us autistic people we will want to do a lot of the stimmy behaviours that soothe and stimulate us and gaming is one such activity.
He may not respond emotionally like you would like him too, but asking him about his interests and what's hard and challenging and giving him empathy and advice that will help him stand up for himself and build some empathy, even if just an intellectual understanding of what is fair and reasonable to expect from him, and other's towards him, will likely help.
