Aside from the good advice given, her insecurity ("doesn't want to be alone") needs to be addressed as well in order for her to not repeat this again with him or someone else. Reasoning with her will only go so far so look into the EmotionCode to deal with the underlying issues that are involved. Hope things turn around soon.
Sadly, most abusers exercise a mental, as well as physical, control over their victims, making them believe that they are the ones at fault, not the abuser. It seems to me that your mother-in-law has fallen victim to this, which, combined with her fear of loneliness, is keeping her in thrall to this man. I think it is unlikely that her family will be able to convince her of this, as she will vigorously defend him. Is there a way by which her children can set up some kind of alarm system so that she could summon help quickly if she is in trouble?
Go to your nearest Domestic Violence Agency and sit down with an advocate. They will have a great deal of information about the signs. Isolation is a great tool of the abuser as is silence. Find out if the abuser is tracking your mother's phone and if she will be safe going out with you or your siblings-in-law. If he is tracking her phone or they are on a family plan, have her turn off the phone when she does. Make sure she is safe. Seek help soon, if possible.
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