Your parents are divorced. Your mother has been remarried to your step father for going on 18 years. Your step father is an alcoholic, horribly controlling and emotionally and verbally abusive. He is abusive to everyone but particularly you and your mother. He is usually good with the kids but one day, he finally crosses that line and, with his words, tears your kids down and leaves them in tears.
Cutting this man out of your life means also losing your mother, brother, and two nieces. You're a single mom, all you have left is your kids and your biological father who will stand by you.
Would you cut your step father out of your life knowing it means also losing your mother and brother?
*ETA* For clarity, losing your mother, brother and nieces would be their choice, not yours. They would likely refuse separate visitation.