One plot could be Naruto protecting his little sister from the threats that still lurk in the ninja world. Since he has grown older and more experienced, he uses his skills to keep her safe.
Maybe Naruto's little sister has some unique power that she can't control well at first. Older Naruto then takes it upon himself to train her, just like how he was trained by others. They could have adventures during the training process, facing various challenges together. And through these experiences, their bond as siblings becomes even stronger.
In some fanfictions, Naruto's sister could be in a coma due to a curse. Naruto discovers a rare herb or a special chakra technique that has the potential to wake her up. He has to overcome many challenges, including political opposition within the Hidden Leaf Village who think he's wasting resources on his sister.
One possible plot could be that the older sister is a powerful witch who has been secretly protecting Harry all along. Maybe she has her own network of allies within the wizarding world that she uses to keep an eye on him. For example, she could be in contact with some of the more reclusive magical creatures who provide her with information about Voldemort's movements.
One possible plot could be that Lemon, Naruto's older sister, helps Naruto train in some unique jutsu. Maybe she has some special knowledge passed down from their family that she imparts to Naruto, which gives him an edge in battles.
One interesting plot could be the sister having unique powers that she has to hide from the Akatsuki at first. Maybe she has a special ability to heal, which is both a blessing and a curse as the Akatsuki might want to use it for their own ends.
She could also be a key figure in the political intrigue of Camelot. As Arthur's sister, she has a certain status. There might be those who try to use her to get to Arthur or undermine his rule. So she has to navigate the dangerous world of court politics, using her wits and charm to outwit her enemies and protect Arthur and Camelot.
One possible plot could be that the sister gets into some serious trouble in the Vampire Diaries world, like unknowingly making a deal with a vampire that endangers the family. Her older sibling decides to spank her as a form of punishment to teach her about the dangers. Another could be that she breaks a family heirloom that has some sort of protection spell in the vampire - filled town, and the spanking is more about her learning respect for the family's heritage in this supernatural context.
One interesting plot could be her going on adventures with Natsu and his friends. She might have unique magical abilities that help the group in their quests.
One interesting plot could be Hisoka's sister getting involved in the Hunter Exam. She might use her unique skills, perhaps some that are similar to Hisoka's but with her own twist, to navigate through the challenges. For example, she could have a special ability to read opponents' intentions like Hisoka, but she uses it in a more empathetic way to avoid fights when possible.
One interesting plot could be that Gaara's little sister has a special power that she has to keep hidden from the village at first. This creates a lot of mystery and tension as she tries to navigate her life without being discovered.
One possible plot could be that Naruto's sister is seen as more talented or important in the ninja world. For example, she might be a prodigy in a certain jutsu that Naruto struggles with. So, the village focuses all its attention on her, leaving Naruto feeling ignored. Another could be that she has some special power or connection that makes her the center of attention, and Naruto is constantly overshadowed by her in every aspect, from training opportunities to social recognition.