She could be on a quest to find a long - lost family heirloom that has the power to either bring great peace or great destruction. Inuyasha and Sesshomaru initially don't believe her, but as she uncovers more clues, they start to take her seriously. Along the way, she befriends some unlikely allies and has to fight off those who want the heirloom for their own evil purposes.
A good plot could be that their sister is a mediator between Inuyasha and Sesshomaru. She tries to bridge the gap between their constant feuding. Maybe she's the only one who can convince them to work together against a common, extremely powerful enemy. Also, she could have a forbidden love with a character from an enemy clan which would add more drama.
One plot idea could be that their little sister has a unique power that both Inuyasha and Sesshomaru don't fully understand. This power could attract various demons who either want to use it or destroy it. Another idea is that she gets lost in a different realm and has to find her way back while learning about her own abilities and the relationship between her brothers.
You can also build a plot around her relationship with Sesshomaru. Maybe there is a rift between them that needs to be healed. She tries to do something to regain his trust or respect, but things keep going wrong. This internal family drama can be intertwined with external threats like demon battles.
One idea could be to give the sister a unique power. For example, she could have the ability to control the elements, which would set her apart from Sesshomaru and Inuyasha. This power could also create interesting storylines as she might have to learn to control it.
One idea could be a story where a powerful magic affects both Sesshomaru and Inuyasha, causing them to be in a situation where they have to deal with the unexpected pregnancies. This could lead to them having to put aside their differences and work together to understand what's happening to them. Maybe they go on a journey to find a cure or a way to handle the situation, all while facing various challenges from other demons who see their condition as a sign of weakness.
Start by deciding her backstory. Maybe she was separated from her brothers at a young age and grew up in a different environment. This could influence her values and beliefs. Then, think about her relationships. Does she look up to one brother more than the other? How does she interact with other characters in the Inuyasha universe? Also, give her flaws. If she's too perfect, she won't be believable. Maybe she's a bit too trusting or has a short temper when it comes to protecting her family.
In fanfiction, the baby could be the key to unlocking some long - lost secrets in the Inuyasha universe. Since Sesshomaru and Inuyasha have such complex histories and lineages, the baby might hold the answers to ancient prophecies. It could also have a special bond with the Jewel of Four Souls, perhaps having a unique way of controlling or protecting it that neither of its parents had. And of course, there would be all kinds of emotional drama as the two brothers - turned - co - parents deal with their new relationship.
One plot idea could be that Sesshomaru has to raise Inuyasha in a world where all the other demons are against them being together. He has to teach Inuyasha how to control his powers while also protecting him from various threats.
One popular 'little inuyasha and sesshomaru fanfiction' could be 'The Bond Beyond Rivalry'. In this fic, it explores the unexpected moments of understanding that develop between Inuyasha and Sesshomaru when they are in their younger forms. It delves into how their different personalities interact in a more innocent way compared to their adult selves.
Another fanfiction concept could be set in an alternate universe where they are all students in a school. Inuyasha might be the troublemaker, Kagome the studious one, and Sesshomaru the aloof and brilliant student. There could be a mystery at the school that they have to solve together, and through this, they form an unlikely friendship.
One common plot element could be the growth of the relationship between Sesshomaru and Inuyasha. For example, at first Sesshomaru might protect Inuyasha out of a sense of duty or family obligation. Another element could be the challenges they face together. Enemies from their past might resurface, forcing Sesshomaru to use his powerful abilities to keep Inuyasha safe. Also, there could be a development in Inuyasha's character as he learns to trust Sesshomaru more and more.
One main theme is the exploration of family and rivalry. Since they are brothers, there's always that underlying family connection despite their rivalry. Fanfictions often play with this dynamic, showing how their relationship might have been different in their youth.