It's almost strange, how comfortable everything is.
Anna moves out of her room and shares with Elsa. It's helpful to have another person there when Elsa's back hurts too much to move, or when she's so parched she feels like she might die. Having Anna there... is wonderful.
And it's not all about Elsa. She's there for Anna, too, when Christmas gets close and Anna's tormented with memories of her childhood. When she's dessperate for a hug or cuddle because her 'human contact' tank is running empty.
They both help each other.