MC: Cassey had nobody but me.
Nicole: Yes, I am alone. I was always alone when you were not with me.
Nicole: Do you really think Mom could replace you? She died, and I am still alone because you are not here!
The way I interpreted this was that their mother was alive 5 years ago when MC left, but died in the interim. This could mean a few things:
- The author either had a different idea or just a vague idea of past events when writing the scene.
- The author/translator accidentally wrote it in such a way to give an incorrect impression.
- The author intentionally wrote it in a vague and misleading way to set up the scene at the end of this update.
- Their mother survived for a time after the shooting.
As for which option is correct, your guess is as good as mine.