I have not experienced your situation but I have experienced an extended time when our son was not communicating with us. I understand that you can’t use LEAP when there is no response! However, this illness typically changes over time. We (both parents and a sibling) were prepared when we DID have the opportunity to use LEAP months later during a hospitalization. Even when our son was often non-communicative and was in a wheelchair while I gave him a back rub, when I simply said “What you think is important to me”, I couldn’t believe what an immediate difference that made in his demeanor. Of course, he was a captive audience in that scenario! But this opened the door for him to be more cooperative. And it was still a long journey of ups and downs, but today he is on medication and SO MUCH BETTER.
The best thing I can suggest is to not give up. Be as creative as possible. You could try leaving written notes, for example, mailing cards or letters, or leaving a voice mail message if she’ll possibly listen to that. When our son was missing for 3 months, I gave up on voice mail because his phone was full so I texted him every few days. He never responded the entire time except ONCE and this is exactly what he said “Hey Mom,I just want you to know that I love you very much! I’m not mad at you.” I understand our son’s demeanor is totally different from your sister’s, but again, I encourage you to think creatively and try to be patient. And have HOPE that things will change.