I think that for you, it's better to be around her more often.. and make sure that you are very comforting with her (give her hugs ) and make sure to tell her the simplest things like " Well, no matter which way the sun sets ... even that isn't as absolute as our siblinghood. You're my sister, and I want you to remain my sister forever because I love you as who you are " and remember to keep things simple.
Perhaps what she needs are the simple things in life. The most complicated problems usually have the most simple answers.
You're a really good person and I'm proud of you!