The challenge here for you, and more significantly your family, is that this man feels threatening to your family culture and unity. By rejecting this man, you risk alienating your sister, and this is why the other family members tolerate him. They hope that by sticking together, they will be able to keep their sister and daughter close, and perhaps they hope he will change if he can be enfolded into the family.
I think this is understandable and may be effective in helping to protect your sister from a man that sounds like a potentially abusive person.
Have you, your other sister and parents ( and your partner if you have one, you don't say) been able to discuss this together? Building solidarity between you could be very important, not in order to criticise your sister's choice of partner, but to agree a way to handle his disrespectful and controlling behaviour. For example, if he speaks to your son again in this way, what will you do as a family? It should not be down to you alone.
Of course an aggressive confrontation isn't helpful ( much as you would love to punch him, (I know I would if someone spoke to my child in that way!)-remember if you get to this point, it will turn into some kind of alpha male thing and dis-empower everyone else from getting involved. Some seriously assertive boundaries do need to be set, such as " In our family, it's not acceptable to speak to each other like this". This kind of message will need to be repeated like he 'broken record', and not just by you. Encourage your sister and parents to firmly and calmly shut down his dominant behaviour.
I hope that together you will be able to support your sister and keep up your family values of respect towards each other.