HappyChappy - first it's wonderful that you have taken the steps to start recovery from what was no doubt an upsetting childhood.
As far as your sister goes. I can understand your wanting to have a point of validation and a connection with your FOO. It is so helpful to healing to have someone who was there to confirm events and empathise with the effects, however she may simply be unable to do that.
Me I was aware that my uBPDm's behaviour was unstable at best but it never occurred to me that this was different from other families. My brother was aware from a very early age and left as early as he could. He tried to talk to me about it many times but for 20 years I disagreed with him. I was so caught up in defending my mother and believing her representation of HIS bad behaviour that I simply didn't accept it.
What I am saying is that your sister may be finding it difficult to gain a perspective that agrees with yours. Or, and as you may already be understanding, undertaking a journey that reveals SO much about a parent - child r'ship gone wrong, she may simply not have the strength or desire to start down that road.
In any case, although it would be helpful to you to get her validation, it's her choice how she behaves. Even if she idolised you and you did things for her, it is still up to her what she does.
Your main focus may have to be on your own recovery.
Have you read through any of the material on the L5 board?
I found this link particularly helpful:
https://bpdfamily.com/message_board/index.php?topic=108307.0I hope that posting here and exploring the resources can help you to get lasting benefits.