Oh boy.
My sister and I are in a similar situation, although it's complicated by the fact that she has children as well.
She and I were victims of physical abuse (not by my parents). She, however, endured sexual abuse as well from a family member. It is pity that preserves what little relationship we have with each other, but it's the anger at what she's done with her life that defines the way I relate to her. I know it's judgmental, but anybody who dumps her teenage daughters on two elderly parents with cancer doesn't really garner much respect in my book. I have my own family, and for a while when my parents were undergoing treatment things got incredibly hairy. Fortunately, her ex-husband has them 50% of the time, while she spends the child support he provides on plastic surgery because she "deserves it." The conversations between her ex-husband and me, when they do occur (usually once a year) , resemble the conversations of two soldiers exchanging wartime stories.
I know she's damaged. I know she endured things as a child no child should endure. But I hate how her life has become an unintentional testament to her abuser, and how she's basically modeled her life to be just like him. I hate how she hangs out with the woman who physically abused us, and effectively dismisses my own struggle to be a functional human being. I hate the walking on eggshells. I hate how she surrounds herself with inherently broken people, and I stand back in awe when I observe her emotional assembly line - she charms people to do her bidding and then throws them under a bus on a continual basis. It is telling that her closest friends sided with her ex-husband.
I remind myself of those reasons when people tell me "I should take care of my sister." In my mind, anybody who says that should be willing to pledge their own financial resources to help her. It also just makes me sad that everybody thinks that she's incapable of taking care of herself, her most of all. One acquaintance remarked recently "She has one of the most passive mindsets I've ever encountered."
My allegiance is to her daughters, who have had no say in this whole sorry mess. I'd like to think if she was of her right mind, she would wish me to do what I'm doing now. It's a tragedy to know that she's internalized so much of her abusers' behavior to the point that she drugged up her mind into a state of permanent delusion. If she actually took steps to face what she's done and to do the hard work of healing herself I would be so proud of her. But at this moment I refuse to participate in the illusion that everything is OK. Because it is most definitely not.