She might be one of those that need a Preganacy scare like gestational diabetes or preeclampsia to figure it out.   Not that I wish the worst on your sister but she's going to need a reason to "change" , for the others that she's projected that persona to. She's going to have to decide if being a healthy mother and doing right by her child is more important than an image she's built.   Wish you the best of luck. If anything does happen it's you she's going to run to.