What is the ruling on the sisters of the wife assaulting the questioner's home and beating his wife and mother, accusing his mother of sorcery, and threatening the wife with divorce?

If the situation is as you described, with your wife’s sisters assaulting your wife and your mother by beating and insulting them, and accusing your mother of sorcery without proof, then they are unjust and sinful. Such actions do not have a specific prescribed punishment in Islamic law; rather, they entail a discretionary punishment (ta'zir) determined by the judge, based on what he deems sufficient for deterrence.