Perhaps she's a sister who was thought to be dead but actually survived. She could have been corrupted by the experiences she had while being 'dead'. When she returns, she blames Percy for not saving her properly. This false blame leads her to betray Percy. She could start spreading rumors about Percy being a coward or a bad leader among the demigod community.
For Percy, you can show his protective side more when it comes to his sister. Have him be more cautious and less reckless in situations where her safety is at stake.
One idea could be that his little sister discovers a new, hidden power within herself while on a quest with Percy. Maybe she can communicate with mythical creatures in a unique way that helps them solve a mystery.
Percy would likely be very confused at first. He'd probably feel a mix of anger and sadness. He might start to question his own actions and wonder if he did something wrong to make his sister leave.
One possible plot could be Percy feeling a deep sense of betrayal and loneliness. He might struggle with his self - worth as he wonders why his sister left him. This could lead him on a journey to find answers about his family and his own identity, perhaps even uncovering secrets about his demigod heritage that his sister knew and he didn't.
The sister could be under the control of a powerful god or goddess who has a grudge against Percy. She might be forced to betray him. For instance, she could lead Percy into a trap set by the god. She could lie to him about a mission, saying it's a rescue mission but in fact it's a trap where the god is waiting to take revenge on Percy for some past deeds. This would be a really heart - breaking betrayal for Percy as he trusts his sister.
One way is through facing challenges together. For example, when they are on a quest, they might have to rely on each other's skills. Percy's sword - fighting skills and his sister's magic (if she has any) can complement each other. They also learn about each other's weaknesses and how to protect one another. In addition, they can have heart - to - heart conversations during quiet moments in their adventures, which will deepen their relationship.
In the fanfiction, their relationship could start off rocky. Maybe the sister resents Percy for always being in the spotlight. However, as they face challenges together, like fighting a powerful monster, they start to understand each other better. Percy learns to listen to his sister's ideas, and she learns to trust his experience. Eventually, they become a great team and have a really close relationship full of mutual respect.
A common theme is the discovery of the sister's powers. For example, she might find out she can control water like Percy but in a different way. This adds an interesting twist to the story as they learn to use their powers together.
I really like 'A Sister's Tale in the Percy Jackson Universe'. In this fanfiction, the sister character brings a new dynamic to the story. It shows how she copes with the knowledge of the Greek gods and her relationship with Percy, which is full of both love and typical sibling spats. It's a very engaging read.