My first response would be to talk to your sister when she's sober and tell her that you love her and want to have contact with her, but you can't talk to her when she's been drinking.

I've tried... I guess I have to keep on trying ... I dont want her to feel she cant ring me bacause she needs support now more than ever... its just difficult

Before my father committed suicide, he used to call me drunk and tell me dirty jokes. My friend said she knew it was him because she would watch me curl up into a fetal ball.

I'm sorry that happened to you - my dad used to ring me every weekend - he would say terrible things about other people - I would tel him to stop and he would for a while but then go back to it - one he had cancer - it got worse .... I guess thats when I first got afraid to answer the phone..

Since you are aware of your own PTSD recovery process, you have to take care of yourself. You aren't doing your sister any good if she causes you that much stress that you're afraid to answer the phone.
I know, I'll have to try and discuss it again with her without her feeling i am pushing her away

Do you have caller ID? You could at least know whether she's calling, and then be able to decide if you feel strong enough to take her call--not knowing if she's drunk or not.

cant not answer the phone - could be an emergency -

If you're not careful, you could end up being a co-combatant with her.

co-combatant? - do you mean someone she rings to pick a fight with? I watch what I say but I can still get into trouble and sometimes when im low - i dont watch my words as much as I should
Keep us posted on how things go for you.
Wil do -
I cannot have any contact with any of my family of origin. None of them.
Humpty dumpty fell off the freakin' wall and no amount of kings horses and therapy are going to put our family back together again. Sad, but true.
My family make me completely crazy.

somtimes we have to cut our loses - but it still hurts - I guess i was always the peacemaker - always wanted to play happy families - till my eyes were opened - then I realised that was a dream that was never real and would never happen

Right now, my healing is my number one priority.