IF YOU DO NOT DO SOMETHING NOW YOUR CHILDREN MAY BECOME PART OF THE JUVENILE COURT SYSTEM!
Since you've known about the abuse for weeks have you reported it to anyone? Has your son made any statements to doctors or teachers about the abuse? Have you taken your son to a counselor (with his father's permission)?
You must seek ex parte relief to prevent the father from allowing the step-mother to be near the children. Since the court will not have any jurisdiction to make orders against step-mother they will have to use the father to "control" who the children spend time with.
Any delay on your part may be viewed by the Department of Children and Family Services as neglectful in allowing your children to be victims of Domestic Violence.
While the laws in California are much different than North Carolina, it appears that you should seek Ex Parte relief immediately. As their mother, you can file on behalf of your minor children. Make sure you consult with an experienced, local attorney with extensive experience in Domestic Violence and Family Law.