I give up completely on my family.

The only people i can count on 100% are my children and my SO.
I agreed to meet my mother's other daughter yesterday at my mothers house with the hope that we could atleast have some form of closure and be civil for my mothers sake.... i didn't see the point as Sarah (my twin) won't listen and it'll be everybody elses fault, but i agreed for my mom as it was hurting her.
Sarah arrived and took out a pen and paper to take notes and had a list of things I've done wrong. I tried to stress that her behaviour is unacceptable (see the previous blog about 7 days of hell that was suppossed to be a holiday) and she has behaved the same way all our lives. My point was immediately dismissed by her as it's the past and i should "get over it" yet when asked what it was that i had sone wrong to deserve such hatred.... she said it was unfair that covered our shared bedroom with posters and didn't apologise for throwing her keyboard on the floor (i dont recall that ever happening and my mom would have MADE me apologise). So, i can't use past behaviour to make a point about her current behaviour yet she can throw that at me? She believes everything my ex (who's been going to her house the whole time) and asked my mother if she had seen anything in writting about what i claim he's done because she doesn't believe me, She's lied to me and mom and we've caught her out in her lies. She admiited that she didn't want to give her 2 little boys the birthday presents i'd bought because i didnt take them myself and my time was more important, when i counted asking if she thought it was better that they think i'd forget them completely she shrugged and changed the subject saying it didnt matter now.
Has anything changed? NO
She's got away with it yet again, purely because she started crying when my step dad (god bless him) shouted that she had a problem and needed help out of pure frustration.
Did she hate him after that? NO! she was over friendly when we went to the pub (he and i needed a drink and something to eat) she acted like nothing happened. Whenever i tried to talk about my SO (when my mom and step dad asked about him) she rolled her eyes. Everytime i tried to talk, she turned the conversation about her. My mom told her i'd found some pictures of the dog we owned growing up "I've found some pictures of him too" when asked what pictures she changed the subject onto a neighbours dog.
What choice have i got?

Is it OK for me to get hurt this way because i get up,dust myself off and carry on? I feel that my mom wont tell Sarah how she really feels because of how she'll react yet i have to suck it up? I feel i have to let my hurt go just so Sarah doesn't feel bad and face the truth and continue living in her bubble where I'm a piece of shit sister who in her mind did nothing for her, I'm a terrible mother and my SO has his claws into me so deep that im turning my back on everyone. I'm too much like my Dad (who she doesnt get on) she doesnt talk to out brother either.
Thinking about it, put all 5 of us in a room...

I'll get on with my mom,dad and brother,
My brother will get on with my mom,dad and me
my mom will get on with sarah,me, brother and my dad.
Sarah will get on with my mom.

Can no one see what me and my step dad see?

I give up, whats the point in this "family"?