Well, this story could be a simple family - themed one. The sister might be helping out because their parents are busy. By putting the boy in a diaper and dressing him, she is playing an important role in the boy's daily care. It could also imply that they have a close relationship and she is like a second mother to the boy in some ways.
He might feel a bit embarrassed if he is old enough to understand that diapers are for younger kids. But if he is really young, he may just feel comfortable and safe with his sister taking care of him.
It might be a lesson about empathy. When the boy is put in a dress, he may experience the kind of stares and comments that girls often face when wearing certain clothes. This could make him more empathetic towards the female experience.
Once upon a time, there was a little boy named Tom. His mother put him in a diaper as he was still a baby. Next door lived a girl named Lily. She loved wearing her pretty dress. One day, their families had a picnic together. Tom was lying on a blanket in his diaper, looking at the sky with his big innocent eyes. Lily, in her dress, danced around the picnic area, making everyone laugh. It was a simple and sweet day for both of them.
Maybe it's a story about a school play where the boy has to be in a dress for a role. For example, in a play about a fairy tale where there are no enough girls to play all the female characters, so the boy is put in a dress to play a princess or a fairy.
There was a boy who often wet his bed. His mom, tired of constantly changing the sheets, got him some diapers. One day, for fun and also to make the boy feel less self - conscious about the diapers, she found an old dress in the attic. She put the diaper on the boy and then the dress. The boy was initially shocked. However, when he saw how his mom was smiling and trying to make the best of the situation, he started to laugh too. It became a little family joke, and over time, with the help of the diapers and his family's positive attitude, the boy's bedwetting gradually improved.
It is far from normal. This action blurs the boundaries of appropriate behavior. A sister should respect her brother's identity as a male. While children may engage in some innocent role - playing, this goes beyond that. It could potentially cause psychological discomfort to the brother, and it also doesn't align with the values of respecting individuality and gender differences in a family environment.