my post below was in response to the same thing px addressed, and echoes the meat of what was said, but without the mean.
after reading this post originally, i went and found this thoughtful and tactfully expressed message on a domestic violence forum, and had not had an opportunity to get back and post it in til now.
I am very sorry you and your family find yourself in this situation.
I gently suggest that this is one of those complicated scenarios best left to experts to help you work through. The advice above to tell him not to come, dump him, keep the violence away from mom, bring the violence to mom, tell the guy off, tell your sister to do this or that, etc. could very well put your sister in more danger than you could ever imagine. It could take her life, no hyperbole intended. This is not an area in which armchair diagnoses from amateurs will help you come to a satisfactory conclusion.
Please call the domestic abuse crisis hotline in your area and talk to a trained professional. They are equipped with the knowledge and training to help you deal with the decisions that will best protect your sister and your family.
What I learned from years of training and then working with domestic abuse victims: Often, what seems like poor decision making or weak, stupid women caving in to an abusive partners' demands are women doing what they need to do to stay alive and safe.
More often than not, victims are best positioned and best educated about the whims and dangers of their partners to make decisions to keep themselves safe. DTMFA is not always sound advice when the MF is abusive. Especially when the advice is based on 'gut feeling' of well-intentioned internet strangers armed with anecdotes. This kind of advice can unwittingly escalate the danger the victim finds herself in.
That's somewhat tangential to the question. The correct answer is to seek professional help from a domestic violence advocacy group, hotline or shelter immediately. Someone will take your call now, at midnight or on Thanksgiving Day. Emergency housing can often be arranged for your sister and her children at once if necessary, depending on where you live.
Christmas season or not, mom's health or not, the most important thing is the immediate and continued safety of your sister and her children.
Best of luck.
this advice obviously related to a more overtly abusive scenario, but the message is still good.
it's a terrible shame that your sister's situation is compounded by her illness. did she get diagnosed after she had spent some years with her current husband? are her doctors aware of her addiction?