Hey girl!!!
Your sister is trying to manipulate you. She knows her daughter will get upset, and that hopefully that will make you break down and submit to her. The more you give in, not only the more she wins, but the less reason she has to take rehab seriously, because she knows that as soon as she gets out, she'll be able to get whatever she wants out of you.
Please don't let yourself feel like you have to go see her. She's a big girl, and she has people there at rehab that care about her. She is not alone in this like she tries to tell you she is. In fact, IMHO, not seeing you may be just what the doctor ordered. Maybe it'll click with her that you're tired of her treating you this way. And if not, then she needs to be able to focus on herself anyway... because she's got a lot of work to do.