Sorry to hear that your fears on the topic were confirmed and that she wasd talking about her own hardships. I'd try to get her to open up about what exactly is hard for her about it and see if there is anything you can do to make it easier for her.

This all really puts you in a bad spot of having to deal with the BP AND figure out how to maintain your relationship with your sister. She needs to open up for you to know how its affecting her.

I hope it all works out!