You're so sweet to be concerned, and bless her heart she just misses her sister so much. I think she wants to go and be with her sister, so here is what I would do. I would sit down with her, and tell her you know how much she is hurting and missing her sister. I would say (name) is looking down on you and knows that when the time is right, you two will be together again. But she doesn't want you doing anything to speed this up, because she wants you to enjoy life at least as long as she did. It would sadden her so much for you not to do this. We understand how you feel, but you've got to think of how she would feel, and take good care of yourself for HER. You must eat well, and take all your medications so that you can do this one last thing for HER. You have a chance to make your sister smile down on you, don't throw this away, she needs you to be strong for HER.
I think she will do anything for her sister, and this way you aren't saying outright that she isn't taking her medication, just reminding her how important it is that she does. Then see if she starts taking it again. If not we'll need a different approach. Tjis shouldn't put her on the defensive because you aren't accusing her of not taking the meds. It's very good that she is journaling especially at this time. I hope this helps.
Maybe you can see her doctor next time she goes and explain the situation. Her doc may be able to advise you how to cope. Who does she live with? An elderly person can be very difficult for you to help on your own.
My elderly mother lives in another part of the country and thinks all the family is out to get her except my brother who is mentally ill. There is nothing we can do to help her out of the mess the two of them live in.