Oh my god...
It's never good to lose someone close to you. Never. I still don't know how to deal with it right. I lost family members left and right over my years of growing up, my family turned to shambles, and recently last year my little cousin Larry (10 years old at the time) had gotten a cancerous tumor in his head. Thankfully the doctors removed it early enough in time and eventually he had a spoiled spoiled Christmas, but if things had gotten worse, I wouldn't know how to deal with it: He's very close to me as if he was in my own family.
I don't know what to say, but I'm still speaking as I go... But keep in mind, I'm no religious person. I don't know how preying will work from a person like me, but my condolences and feelings go out to you. Just remember her with your memories, but don't dive into the past too often, that'll only hurt you. Or probably that's just in my case. *sigh* In any case, look forward. I don't know your sister, but I'm sure she's like many others, that she'd rather want you to be happy. Just don't let this bog down your entire life, you have to move on as do we all, no matter what tragedies occur. As your signature quotes, pessimism isn't a survival trait.
I still don't know if I'm helping or not, but keep in mind, my feelings go out to you.