"We are sisters, remember?"
"We are always going to be together, even if i am no longer a human, i'll protect you...Y/N"
Memories are a interesting thing, because, most of the time, you can't control them. You just think that you can. That's why you don't remenber almost anything about you, only one thing, you know, you have a sister, but you never saw her face.
These are the two things you remenber.
You are now alone, in this dangerous world where you have no idea on what to do. So listen to the voice in your head, let it lead your path until you find a purpose. That's the least you should do now.