Also... I agree with Todd. And I can imagine your frustration as effectively being head of the extended family, the responsibility, but can't stop things from sliding down hill. I think you really do have to look after yourself in this situation and not be dragged down into having a fight on his terms - this is where you get hurt/assault conviction/jail
And your sister, the niece and her boyfriend will all hate you no mater whether the outcome is good or bad.