What snowybaubles wrote.
What do YOU want to do for Christmas?. Forget your mother, sister, dad, your H and his wife in all this for one second, what do you actually want to do?.
Where are your boundaries at here with regards to all these people?. They seem so low as to be almost non existent particularly when it comes to your family like your dad, mother and sister. If you want to see your dad then make arrangements to see him particularly if he is in the same area as you. Do not apologise to your mother and sister on his new wife's behalf. What have you got to apologise for, you remain their whipping boy here. These two as well have never apologised nor have accepted any responsibility for their actions either have they?.
Its really of no surprise now your DH has decided to have no further contact with them. He has had more than enough of them treating you so abusively here and he is well within his rights to walk away. Ultimately you will need to have no contact at all with your mother and sister.
re your comment:-
"My mum does not spend xmas with us but my sister does so its about making amends with her for my dads sake! We have always been a close family and i have sucked up my mum and sister bullying behaviour towards me for an easy life."
Being an appeaser does not work as you have sadly found out. It just makes them worse and they know you are weak so go in more for the kill. I also feel you've been trained from early childhood to put your mother and in turn sister first, with you and your needs dead last. No wonder they've walked all over you, they did this to you and sadly you were conditioned also into accepting this role.
The close family image that you adopted for your own self was and remains a myth. You are also enveloped still in fear, obligation and guilt and these along with your own inertia re your family of origin keeps you enmeshed within it.
People from emotionally unhealthy and otherwise dysfunctional families end up playing roles and your role here assigned is scapegoat. Your sister is your mother's carbon copy and remains favoured (as do her children, your kids will also be scapegoated by them). You are very much the scapegoat and besides which your father failed you here when he was with your mother by failing to protect you from his wife's excesses of behaviour. He acted out of self preservation and want of a quiet life and threw you and in turn your sister under the bus.
Your dad's new wife here is the flying monkey and she is acting in her best interests here rather than yours. She is also manipulative by using your dad's ill health in such a manner i.e. to tow the party line. She had no right to say what she did to you re your mother and sister and therefore her opinion should be ignored.
Do read "Toxic Parents" by Susan Forward as a starting point and have a look also at the current November "well we took you to Stately Homes" thread on these pages.