Oh your poor sis Cod.
What a farcking knob. It is very hard to understand why we let them wear us down and we end up agreeing to the unimaginable and self doubting all the time. It happens over years and you just accept it, then think it is normal.
It usually takes a massive jolt for it to come to light, walking away is often the easy bit. It is maintaining the split that is hard.
Fantastic she is off on holiday with you, just what she and the children need. I am sure you will remain strong for her and it is good that you have a relationship with him and especially your H so that you can tak to him too. Be the voice of reason as it were and field him from your sis.
Like you said no point you all screaming and shouting at each other, it does not help. H sounds great btw, so do you.
Your mum hugging her was lovely, wish mine had done that. A hug from yyour mum is really all you need. That and a kick ass sister standing your ground