I have two sisters, I'm in the middle, they don't talk to each other and my elder sister has also cut contact with my mother, my father died some time ago.
I was 'used' by all three as a conduit to vent their respective spleens for years until I put my foot down and said 'no more'.
Now I am a conduit for the three of them to share information - since they won't talk to each other.
I have regained 'power' and if any of them ask me to do/say anything I'm not comfortable with I tell them and to be fair to them they respect me for it.
It strikes me, now we're several years down the line with this is that, like small children, they just needed boundaries. My father always maintained the boundaries and when he died it was an extremely unpleasant free for all for about two years until I stepped up and said 'no more'.
So, my advice to you is stand up to them, say no, stick to it, from my own experience it was incredibly easy once I did it.
Don't be the family scapegoat - don't accept the role. You don't have to be their friends, but if they want to be your friends they have to be friendly to you.
Good luck