The therapy is key for you and should be your absolute priority.
Lots of people have challenging relationships with siblings and parents, but very kindly meant, you sound as if you are hugely preoccupied, and triggered by them, far far more than is normal.
You have your own life in a different country yet allow this to dominate how you feel.
Your sister is wisely pointing this out to you.
So therapy would help you get to the root of your own mental health, as to why these relationships are dominating you.
You will not do it on your own, so above holidays, meals out, treats, etc., paying for therapy should be your priority.
It will be a huge investment in yourself, your family, your future contentment, to find out why you are so stuck.
Armchair analysis here is that such was the bullying that your parents allowed her to do to you as a young child for years, that you have a bit of CPTSD from it all.
I think you are trauma bonded to your mother from the way she allowed you to be treated so badly.
Therapy could help you break that bond with her and see her clearly......
...a mother who together with your father were really lazy parents that failled their children completely.
Your parents were very selfish to allow you to be treated so poorly when you were so young.
I think as your children grow you will realise that.
Parenting can be hard for sure, but there is no excuse for allowing a child to be as bullied as you were.
You need to find your anger.
Your mother has some cheek behaving as she does, pushing you to have a relationship with your sister when she was allowed to treat you so poorly.
Therapy and finding your anger would help you I think.
Presently you are still stuck, as the scared little girl being bullied by your sister as your parents watch on.
Being bullied at any age has a profound negative affect on a person.
Being bullied within the home, by a sibling, as your parents watch on, must be the worst IMO.
Finding your anger and your voice through therapy will help you I hope.
Ignore the above if you wish, pure armchair!😁