One common theme is responsibility. The little sister has to take on a task related to the older sibling like getting them to do something they should be doing. For example, making sure they study or clean their room.
In a story I heard, the little sister was put in charge of making sure her older sister practiced piano. She would sit beside her and count the minutes of practice. If her older sister tried to stop early, she would encourage her to keep going, and it really helped the older sister improve her piano skills.
One challenge could be getting the older sibling to listen. Since they are older, they might not want to follow the little sister's instructions easily.
Sure. There was a story where the little sister, Sally, had to babysit her older brother Tom who was sick. Sally made him soup and read him stories all day. It was really cute as she took on the role so seriously.
Protect her. If she is being bullied at school or in the neighborhood, stand up for her. Also, set a good example for her in terms of behavior and values. If you are honest, kind, and hard - working, she is more likely to grow up with these positive traits.
In a fantasy story, the little girl is a chosen leader in a magical land. Older boys, who are warriors or mages, have to follow her commands. She has a special power of foresight perhaps, and she uses it to lead them in battles against dark forces. The boys, with their combat skills, support her plans.
This is a rather unusual situation in most cultures. However, in a make - believe or very young children's play scenario, one could imagine a story where a little brother, perhaps in a game of 'pretend family', tries to diaper his older sister's doll as if it were his sister. But in real - life, this goes against typical social norms.
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.
A big challenge can be the difference in interests. The older boy may like more complex and mature things while the little sister might prefer childish games. This can lead to boredom or frustration on the part of the older boy. Also, if there are any safety issues, like if the older boy wants to go out alone or do something risky, the little sister has to be really firm and responsible which can be difficult given the age difference.