I think your sister is fortunate to have someone in her family to whom she obviously feels able to speak about exactly how she is feeling.
I get the idea that, as you see it, the rest of the family have not accepted that her death is approaching?
It must be so hard for you, but I think that, in terms of what you should "do", you are already doing it.
It seems that you have helped her to feel that she can say whatever she needs to say, and this is likely to be the most helpful thing for her. You don't need to be able to make her feel better....just give her the chance to name her feelings. This generally helps us to feel better, when done with someone who really listens and really cares.
I really cannot emphasize enough what a gift you are giving your sister in this role.
When she says she doesn't know what's next, do you mean medically/pain management/mechanics of death etc.-wise she doesn't know what's next? Or do you mean what happens after we die-wise?
If the former, I am wondering if you can convey her questions to the medical team, and they can talk to her? Do they know how she is feeling? It would be good if they did. Sometimes kids have all kinds of mis-perceptions about the process of dying that makes them even more scared than adults.
If the latter...oh boy! Sounds as though you guys don't have a specific religious upbringing.
Did you feel able to explore her fears? Is the "I didn't have time to be good "a fear of hell, or grieving over lost life?
If she thinks she was not good, perhaps, after giving her time to express this, you could gently explore all the good things she has done and been. Kids typically feel enormously exaggerated guilt over minor misdoings...if you feel able to go there, you might feel comfortable in saying that, if god exists, he's a dad.....he knows enough to expect that little kids will make mistakes, and forgives them. She may want to share things she did that she didn't get caught for, so give her space for that!
Re after death...something a lot of people find comforting, is to think of the time before they were conceived. Was that awful or scary? Doh...of course not. So...if there is simply oblivion, that isn't gonna be scary either.
Kids often imagine being there to EXPERIENCE oblivion, which doesn't sound all that good.
Is there a counsellor available for her? It is not right that you should be dealing with this alone. I get that you are seeing someone....is it just you?
Is there someone for the family? Could you ask to speak with the unit social worker and express your concerns to them and get some support?