How/Why is this a metaphor?

Question

Once more in her bed beside the warm cocoon of her sister

Answer 1

Is her sister really a cocoon? Or is the writer using the image of a cocoon to describe the sister?

Answer 2

I noticed it is a IXL lesson so the answer is that she feels snug

Answer 3

PS me replying 11 years later... 🤨

Answer 4

It suggests that Marianne feels snug