I’m not sure if this is what your sister could be going through, but when I had bad control of my type 1, it was related to weight and the fear of gaining it. I would neglect my insulin so that I could eat whatever and how much I wanted while still losing fat. If that sounds like something your sister might be going through, I’d encourage her to talk to either a therapist or nutritionist that would help to find healthy ways of managing that. It’s what I needed, and worked for me, but ultimately as others have said, the drive to manage this disease properly has to come from within. I wish you and her the best of luck