My mother married my father for the money and status, and never cared for him.
When he lost that wealth and status, she divorced him.
She created court cases, trying to take me and my sister away from our father, so that we would never see him again.
Years passed, and she begun to live with another man, though she didn't marry him. My sister hated this man.
One new year's night, an argument broke out, and the man assaulted my sister, causing bruises upon her wrists. I filmed the ordeal.
My mother didn't intervene. She just told the man that I was filming, as though she knew he could get into trouble. She didn't help her daughter against a man who was hurting her.
My mother and the man still live together, and my sister, though spending some months with our father, now lives solely at our mother's house. She seems to be able to cope with the man now.
My mother is manipulative and influential, and my sister has inherited that personality. Plus, my sister isn't highly psychologically introspective, so she doesn't question things often. It's obvious that my mother is influencing her.
I live with my father, and I'm there for him, because he's growing old. Due to my social difficulties, I can't really do much for my father as in buying food and such, but I'm there to offer my comfort and advice.
My mother looks after her own family and is close with them, yet both her and my sister regard my relationship with my father as unnatural and odd. They say I am his slave, when I'm merely trying to be a loving and dutiful daughter, which is normal in families.
My mother has successfully caused an indifference regarding our father in my sister. My sister seems to no longer care for him now.
We are in Croatia, creating a new life, having escaped the consequences of brexit. My father and I live in Zagreb, and my sister couldn't care less about coming out to visit. It saddens me, because I miss her constantly.
My mother is actively hindering her daughter's journey to Croatia. She's trying to split the family, she's been trying multiple times to turn me away from my father, and she's succeeded with my sister it seems. 
Now my father has been diagnosed with a suspected malignant melanoma and parapsorayasis (I cannot spell it) and my sister couldn't care less.
She thinks that I shouldn't share information about my father.
Anyway...
Thank you for reading. It's cathartic to address troublesome things.