My sister and her ex divorced 8 years ago, shortly after they had their third child. Her ex was controlling, adulterous, alcoholic and physically abusive. On the agreement that she wouldn't press charges ( he had a responsible job in the police force) he gave her the lion's share of the proceeds from the house as the children live mainly with her. Things went downhill as soon as my sister got into a new relationship and they went to court over maintenance payments and access to the children. Not only did the court insist his payments should be the lowest possible he was granted the right to have the children every other weekend and an evening in the week. My sister tried to point out that she didn't like leaving the children with him but the judge said that it everyone has different parenting styles and that she must learn to be more accepting. This may or may not have had something to do with the judge being a fellow mason and a drinking friend ... who knows? 
Anyway, over the subsequent 8 years he has sent endless abusive texts and messages to my sister and has said terrible things about her to the children. They have learnt to try to block it out.
The worst thing of all is how he treats the children: apparently they are sissys, they are whiners and need to be turned into men. He bought the eldest nothing but a pair of woollen gloves for his birthday and insisted on endless gratitude. He wasn't allowed to take them back to my sister's house either, note any of the other things he 'bestows' upon them. He has only JUST got round to begrudgingly sorting out a bedroom for the children in his multi-million pound basement ( married again, new wife's ex was loaded -his best friend btw.). Before this they slept on the floor for years. The youngest wouldn't even poo in his father's bathroom for fear of ridicule, holding on for two days at a time, poor thing.
The ex belittles, he thunders, he threatens and he intimidates. He is frighteningly dominant and they go to see him because they are fearful of the consequences for them and their mother. They know that he is their Dad and that maintaining contact is 'important' because he insists that he loves them. This is so confusing for them and is probably the reason why they don't refuse to go. The youngest is horribly anxious and excessively people pleasing as a result.They have both had to have counselling to deal with their fears and 8 years of sobbing on 'that' night of the week. It's such a shame because they are lovely kids, despite everything they've been through.
To top it off, today the ex sent her a text saying that he won't be paying maintenance anymore as he has given up his job in the force but he would like to see them in them more in the week as well as at the weekends. He also has a history of depressive and manic illness and is known for his erratic, angry outbursts. My sister is despairing, knowing now that she will now have to take yet another job ( 3rd) to keep their heads above water, not to mention having to explain to the children that their father would like to see them more. She lives on a shoestring as it is. What is her legal position? What are her rights? What are the children's rights? Can anybody offer any good advice? After years of being exposed to his rage and let down by the legal system, she is at her wits end. She is understandably frightened of him. Any advice gratefully received x