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When someone even a family member acts out inappropriately, it is reasonable to confront that person. It seems that you brother had not done so possibly because of intimidation, thinking your dad was right or not finding anything wrong with the situation.
If your mom wants to be the one doing the confrontation, she could point out to him when he is calm what she had observed and that it had not been appropriate. Now, if your father of what you are saying has a temper problem, he may not find a fault with the way he had acted and may ignore or minimize the situation. After all, a person can correct their own behavior if they accept that there is a problem. If not, the rest of the family members will have to find way to deal with that behavior (which most likely won't change on its own)
Your sister in law and her husband may want to address this with him as well.