Perhaps the sister is a witch (in a fantasy story). She might be trying to protect her brother from some danger. She believes that by turning him into a girl, he will be safer. Maybe there is a war or a hunt for boys in their land, and she uses her magic to change his gender.
One reason could be that the sister has some sort of enchanted object. For instance, a mirror that has the power to change one's appearance and gender when looked into. So, when the boy looks into it by accident or at his sister's urging, he becomes a girl.
One possible moral could be about empathy. When the boy becomes a girl, he can understand the challenges and experiences that girls face in a different way, teaching us to put ourselves in others' shoes.
It could also be that the sister is trying to prove a point about gender equality. By making her brother experience what it's like to be a girl, she wants to show that there are no big differences between the two genders. However, this is a rather extreme way to do it.
One reason could be curiosity. The boy might be curious about what it's like to be a girl, how different clothes feel, and how people would react. Another reason could be to help his sister out. Maybe his sister was supposed to be in a play or an event and couldn't make it, so he stepped in. Also, it could be for fun. He might just want to do something different and have a laugh by pretending to be his sister.
One reason could be for fun. Sisters might find it really amusing to see their brothers in a different, unexpected look. Another reason could be her creativity. She wants to experiment with different looks and her brother is an easy model to start with. Also, it could be for a joke or to create a funny memory that they can look back on and laugh about later.
It could also be part of a reincarnation story. Maybe in his past life he was a girl and due to some karmic imbalance or a sign from the universe, he is transformed back into a girl in this life to learn certain lessons or fulfill a certain destiny. This could involve a spiritual journey where he has to come to terms with his new identity and the responsibilities that come with it.
Another reason might be a personal exploration of gender identity. Some boys may feel that they identify more with female characteristics and may choose to present themselves as girls, either temporarily or permanently. It could be a journey of self - discovery and acceptance.
In some cases, it could be a form of self - expression. Maybe the boy doesn't feel restricted by traditional gender norms in clothing. He might see a girl's clothes as just another form of beautiful or interesting apparel. For example, a boy who is into art and creativity might want to experiment with different looks, including wearing girl clothes to make a statement about individuality and breaking boundaries. Also, in a family where there are shared clothes or a more open - minded attitude towards clothing, a boy might casually put on girl clothes.
In some stories, it could be because of a lack of other options. Maybe in a small town where resources are limited for a costume event, a boy might have to use a baby girl's clothes to participate. Also, it could be part of a prank. For example, a group of kids plan to prank their parents or neighbors by having a boy dress as a baby girl and show up unexpectedly. Sometimes, it's just for the sake of creativity. A boy might want to stand out in a costume contest and dressing as a baby girl is an unexpected and attention - grabbing choice.
One possible reason could be cultural or traditional beliefs in some extreme cases. For example, in a very patriarchal society where having a girl is highly desired for certain family inheritances or religious practices, a family might try to force a boy to live as a girl. Another reason could be related to some psychological issues of the people forcing it. Maybe they have a distorted view of gender and think they can 'create' the gender they want.
I'm not sure if such a situation is common in real life. But if it's a fictional story, perhaps the sister has some magic power. Maybe she accidentally casts a spell on her brother during a family gathering while trying out a new magic trick she learned, and this spell turns him into a girl.