I empathize immensely, MatBell. From what you've typed, I share similar circumstances with my brother.

Since you posed the question, "what would you do," I will attempt to humbly tell you...

I conveyed my thoughts, perspectives, concerns and so on to him throughout the years (ten years or more.) I attempted to have several "heart to heart" during those years as well. Without divulging too much about my personal circumstances, I would try to not let her (your sister) bother me. I would try to live my life with the acceptance of my life with or without her in my it and not be too concerned.

If she is someone, who is not "there" for you when support, empathy, help and so on would be helpful to you, she seems to be someone, who is not dependable, empathetic, supportive and so forth in general. As you said, "in tough situations you see who is really there for you." To you, is someone, who communicates, interacts, and acknowledges you when you are well, asks favors of you and does not return the act, expectations of you and does not return the act and so forth a friend?

Regardless of the familial, biological connection, I would judge her as a person according to the unfolding history (past, present and future.) I would learn to become resigned to how she is and might not change how she is as a person and with you.

I try to constantly judge how I am and am not in general, towards others in general and so on. Even though my brother seemingly hardly questions himself, how he is a person, how he is with me and so forth, I constantly question my continual judging, observations and so forth. Time and time again, he proves my perspectives and perception of him to be accurate as to how he will probably be and consistent with how he has been in the past.

I would learn to be almost indifferent to her (your sister) and if the two of you are in each other's lives or not while still being open to the possibility with a relationship with one another, being cordial in general to her and so forth. I would not begrudge her in general, but I also would not attempt to be feigning interest with her. Your life is yours to live and to choose as you will. People in general, who take and do not give/reciprocate, will drain the person and wear on the person and his/her being and so forth. The effort, time, thought, and selflessness/kindness/deceny of the person goes unreturned, unappreciated and expected as a one sided "takeship."

I'd try to live my life, regardless of her, any relationship and so on.