This so sounds like my family 20 years ago. Im sorry you are going through this torture. If I could go back and talk to me back then, my only advice would be to remove myself from the situation and protect my own interests. Trying to hobble along in the middle ground and trying to keep the unpleaseable pleased is an impossible situation. I should have done what was right for me much much earlier.

I hope you don't mind me saying this - but it seems like your mum enforces you to support her, whilst your dad seems completely unsupported. It doesn't sound like your mum and dad work well together --- maybe the route to a happy family, is if your dad is supported in his decision to leave? If they are away from each other, not fighting each other (probably not talking to each other) would it allow you and your siblings to build positive relationships with both of them separately?