Your sister was an angel, and no, I don’t mean in a religious way. In that she cared about you so much and tried to do the best for you that she could.
Your love for your sister is so genuine, and hers for you as well. She tried to give you as good of a start in the world as she could, and she appears to have done at least something right. I had a completely different experience with siblings growing up.
When I was 5 years old I didn’t worry much about death. Then again, I was brought up Catholic, so I sometimes lay awake at night agonizing over whether I would go to hell if I died. Yes, even at 5 years old. It was suffocating in many ways.
I have still never read A Wrinkle In Time. I probably should before my son is of the age to read it so I can discuss it intelligently with him, as well as with my daughter when she is of age.