Sometimes it could be a simple misunderstanding. For instance, the older brother might accidentally break something of the little sister's and she thinks he did it on purpose. In her anger, she attacks him. Or if the older brother is given a privilege that the little sister feels she should also have, like going to a friend's house alone first, she might be angry enough to beat him up in a fit of rage.
Jealousy can be a reason. Maybe the older brother gets more attention from parents or grandparents, so the little sister lashes out at him.
Another reason could be that she doesn't fully understand how her actions affect her brother. She might think it's all in good fun, like when she teases him about his appearance or his hobbies. But to the brother, it can be really hurtful. She may not have developed enough empathy yet to realize that she's being a bully.
One story could be that the older brother took the little sister's favorite toy without asking. The little sister got really angry and started hitting him with a small pillow. She chased him around the room until he gave the toy back.
One reason could be personality. If the little sister is more extroverted and the older sister is more introverted, the little sister might be more likely to take the lead. For example, in social events, the little sister would be the first to start conversations or make new friends, making her seem dominant.
One reason could be that the little brother has been disobedient. For instance, if he was told not to play with a certain dangerous item but did it anyway. Another reason might be that he has been unkind to others, like bullying a younger sibling or a neighbor kid, and the sister feels it's her responsibility to correct his behavior.
One reason could be that the sister is more dedicated in certain areas. For example, if it's about schoolwork, she might study harder. So when she gets better grades and shows it to her brother, she defeats him. And she might shame him by saying something like 'I study while you just play games'.
In some family - centered stories, the little sister is really into girly things like tea parties. She convinces her older brother to participate. She makes him wear a crown and a cape, and they have a 'royal' tea party. Through this, the brother is feminized as he takes part in an activity that is often seen as more suitable for girls. He might even start to enjoy it and become more open to 'feminine' activities in general.
One reason could be that the big brother took something that belonged to the little sister, like her toys or snacks. So she got angry and decided to take matters into her own hands.
Well, in a lot of these fanfictions, Stiles can get beat up due to his own recklessness. He might be so determined to solve a mystery or help his friends that he doesn't think about the consequences and ends up in a dangerous situation where he is outnumbered and beaten up. Also, there could be miscommunication. Maybe someone thinks he has done something wrong when he actually hasn't, and they take matters into their own hands and beat him up.
Well, once my little sister got really mad at me because I ate the last piece of her favorite cake. She started hitting me with a pillow. It was more annoying than painful really. I just laughed it off in the end.
Genetics can be a major reason. Maybe the sister inherited taller genes from a grandparent or a distant relative. Sometimes, nutrition also plays a role. If the sister had a more balanced diet rich in essential nutrients like calcium and protein, it could lead to better growth. Another factor could be physical activity. If she was more active in sports or outdoor activities, it might have promoted her growth.