Will my sister ever learn?

My sister has been with her partner for 13 yrs. They have 4 kids together. the problem is he has done nothing but lie & cheat through there whole relationship & has always been mentally abusive towards her eg he would never let her head nights out unless he was their or if she was out on her own she would have to be back before 12 midnight or he would lock her out of the house! He is also not a great father, they would let my nephews watch movies that are over 18 if they were renting them out. My 9 yr old nephew also said to me few weeks ago that his father told him what the SAW movies were about i was disgusted! He has also been rude to myself and my sisters when he was drunk & then trying to use drink as an excuse for his behaviour. But what really makes me mad is my sister always believes everything he tells her & is very gullible with him & he knows this. She knows we don't like him & is mad with us because we won't speak to him & tells us to say hello & behave at family occasions, she should be telling him that not us but she never thinks he is the one in the wrong. He has caused so many problems in our family its gone to the stage i don't want my sister in my life anymore. Is this the right approach to take? please help?