Since I don't have access to the Wattpad story in question, it's difficult to give a proper summary. However, generally, a story with such a title might be about a relationship where the wife has a role related to providing warmth or comfort in the context of the bed, perhaps in a more emotional or domestic sense.
It's not really appropriate. Beds are very personal spaces. You should encourage her to sleep in her own bed. It can also help her develop good sleep habits and a sense of independence.
In historical context, a 'bed warmer' was often a simple device like a hot water bottle or a pan filled with warm coals used to heat beds before the use of modern heating systems.
This is a more serious situation. You definitely need to involve your parents. They can have a long - term talk with her about proper behavior and the importance of privacy. Maybe set some clear rules in the house about where it's okay to be naked and where it's not.
For a young child, it can be somewhat normal. Young children often don't have a full understanding of modesty and privacy. They may not see being naked as something inappropriate. However, it's important to start teaching them about these concepts as they grow up.
You should immediately address the situation in a positive way. Since she's your little sister, she may be just being innocent or not fully aware of social norms. Take her to a place where she can put on some clothes, like her room. After that, you can have a little chat with her about why we wear clothes. For example, clothes keep us warm and also are a part of our privacy. You can also use this as an opportunity to teach her about good behavior and respect for others' personal space.
First, you should immediately cover her with a blanket to preserve her modesty. Then, gently but firmly explain to her that this is not appropriate behavior. It could be that she doesn't fully understand the concept of privacy yet. You can also tell your parents or guardians about the incident so they can have a proper talk with her about appropriate behavior and privacy.
You can offer her a special treat if she sleeps in her own bed for a certain number of nights. For example, a small toy or a piece of her favorite candy.
It's not appropriate. You should gently but firmly say no. You can offer alternative options like setting up a comfortable sleeping area nearby for your cousin, like on a couch with extra blankets and a pillow. This way, you maintain appropriate boundaries while still being hospitable.