Why does Susie have to wear the hat to school when her sister didn't?

I agree

She was her parents first born and the film made it very obvious that she was the favorite. I was confused when they showed the two younger syblings, especially the sister that seemed close in age with Suzie. Up till that point they made it seem like Suzie was an only child.

The mom very clearly said that the younger sister's hat wasn't finished yet.