She might convince him by showing him the long - term benefits for the village. For example, she could tell Naruto that in ANBU, he can prevent threats before they become major problems.
The Anbu sister could be a powerful and mysterious character. In the fanfiction, she might have unique abilities that Naruto is curious about. Their meetings could be filled with tension at first, as the Anbu are often secretive. But as Naruto's friendly nature breaks through, they start to share their stories.
One possible fanfiction could be about Naruto forming an unexpected bond with Kurenai's sister who is in the Anbu. Maybe Naruto stumbles upon her during a mission and they start to work together, gradually learning about each other's pasts and dreams.
For Kurenai, her character could be developed by exploring her relationships with other members of the village more deeply. For example, showing how she mentors the younger generation in addition to her regular duties. In the context of ANBU, she could be given a role where she has to train some of the ANBU recruits. As for Naruto, if he is involved, his time in the ANBU could show his growth in stealth and strategy, which are different from his usual combat style.
Another idea is a story where Naruto joins the ANBU and is paired with Kurenai for a special undercover operation. They have to infiltrate an enemy organization that is suspected of planning an attack on Konoha. Along the way, they learn more about each other's pasts and develop a strong bond as comrades.
One possible plot could be that Kurenai has some sort of influence on Naruto's decision. Maybe she sees his potential in a different way than others. She could encourage him to join ANBU to better protect the village and its people. Naruto, being influenced by her words and perhaps having a sense of admiration for her, decides to take on the challenge of joining ANBU.
Naruto's adaptation to the Anbu in this fanfiction could be a complex process. At first, he'd probably struggle with the strict discipline and the need for secrecy. But his previous experiences in tough situations would come in handy. He'd gradually understand the importance of stealth and how to use his skills in a more covert way. He might also learn from the more experienced Anbu members, and his own determination would drive him to become a valuable part of the Anbu.
Tenten could be one of the characters. She could be Naruto's friend and also have a bit of a crush on him. Ino might also be present, using her mind - related jutsu to try and understand the complex relationships in the harem. Rock Lee could show up too, providing some comical relief and also being Naruto's loyal friend in the midst of all the relationship chaos.
Some of the more hot - headed ANBU members might overreact and assume Naruto is a threat to the village. They could try to use force to get the stolen item back. However, the more level - headed ones might try to figure out Naruto's motives first. They might consider that Naruto could have a good reason for what he did, like if he thought the item was being misused by the ANBU.
I'm not entirely sure if there is a specific 'Naruto cheated on by Kurenai' fanfiction. Fanfiction websites are full of a huge variety of stories, and it could be quite difficult to find one with such a specific plot. You could try searching on popular fanfiction platforms like FanFiction.net or Archive of Our Own using relevant keywords, but keep in mind that the Naruto fandom has a wide range of story themes, and this might be a rather unusual one.