Without the intent of starting a debate on the subject, I'll offer some thoughts that I personally believe, in the hopes they may help both you and your loved one during a difficult time, ok?
I believe that we as humans are flesh & blood, AND energy! Now, it's a scientific law that energy cannot be created, nor destroyed. It can only change from one form to another. Now comes the big word IF. IF you believe in this, as I do, you realize that death is just a transition from one form to another, in terms of the energy part of us. The flesh and blood cease to exist, but the essence of the person does not. And if this is the case, then you can bet your bottom dollar that your sister isn't really going anywhere.
I had a sister-in-law pass away around Christmas time a few years ago. One of the things her and I loved to do was compete with each other at Wheel Of Fortune. She was quite good, as am I. The race was always to solve the puzzle before the other. Now after her death, I find it ironic that when I periodically, and randomly watch the show, I see the puzzles a little differently. Sometimes, it's as if the puzzle says something that's relevant to my day at the time. And when that happens, I'm like, "oops, there's Michele again, saying hello". It's hard to explain in this kind of format, but I feel her presence all the time. As if she's right here with me (and my wife). It's weird, but it's quite comforting at the same time, to know that she's still here, still around us, still having an impact on our lives.
Before your sister passes, have a talk with her. Ask her this. "Sis, when you get to the other side, if you're able to, what kind of sign or signs will you try to send me, that I'll know it's you?" It'll sound like a weird question to her, but talk to her with sincerity. Tell her to open her mind to the possibility that this might be the case. None of us really know, but there's no harm in trying, huh? Then, once that terrible time comes and goes, look for those signs she mentioned. You'll likely be amazed at the frequency with which that particular sign or signs occur.
What kinds of signs? How about keywords. You know we all have "jargon" with each other. You know, like "wassup!", or "go ahead, make my day", or "yeah, right", or whatever is relevant between the two of you. How about places you like to go together, or food you like to eat, or anything that's special between the two of you. Like I said, for my sister-in-law, it was about Wheel Of Fortune. And the puzzle solving. Now, the puzzles are a means of her communicating with me from the other side.
You're in a unique position here, because your sister is still alive (I hope), and you can kind of PREPARE for this kind of situation into the afterlife, rather than trying to figure it out once she passes.
Lastly, let me give you a website. Again, OPEN MIND, ok?
http://www.johnedward.net(To the Admin, I know links aren't allowed, but I hope you'll be understanding to the nature of the submission here).
I'll be praying for you two.
Mike - God bless you both.