I would just give her a card designed for a sister. That's very platonic. And write something like "just wanted to let you know that I'm glad I could have a sister like you." And don't forget the first sunday in August is Sister's Day.
Having said that, you said in your last post "if I were to do something like that now..." That makes me wonder about what has gone on that you haven't written about. What is the "now" part that has changed your relationship? It sounds like in your original posts that she was blissfully unaware of your feelings and that this is your own personal, silent struggle. Has anything been said or acted on? If it has, then this probably won't work. If she has no idea about your struggle, then it should be taken as something very nice. Of course with her having a delusional disorder, that makes her a little unpredictable as well.