Another way is to depict her as submissive in the pursuit of a greater cause. Maybe she is part of an environmental conservation group. She follows the lead of the more experienced members, listens to their advice on how to protect the environment, and is always ready to take on tasks assigned to her. This submissive behavior is positive as it contributes to a worthy cause and can also show how teamwork and a submissive attitude can lead to positive change in the world.
The sister can be submissive in terms of her values and beliefs. Let's say she belongs to a community with certain moral and ethical guidelines. She adheres to them wholeheartedly, not because she's forced but because she believes in them. This submissive attitude towards positive values can be a great example in literature. It can show how an individual's commitment to good values can influence those around them, like her siblings or friends who might be inspired by her.