I'm not sure of a specific 'naruto deidara is naruto's brother' fanfiction, but generally in such a concept, the fanfic could start by exploring how they were separated at a young age, perhaps due to some sort of conflict between the Hidden Leaf and Iwagakure. Then as the story progresses, they could discover their relation in a very dramatic way. This could then lead to them having to choose between their existing loyalties (Deidara to Akatsuki and Naruto to Konoha) and their new - found brotherhood.
One possible plot development could be that they initially fight against each other as enemies, not knowing their relation. Then, through some kind of accident or revelation, they find out they are brothers. This could lead to a lot of internal conflict for both of them, especially for Deidara who has been on the 'wrong side' in the eyes of Konoha. Another development could be that there is a third party who knows about their relation and tries to use it for their own gain, forcing Naruto and Deidara to work together to overcome this threat.
Well, in this fanfiction scenario where Naruto is Deidara's brother, it would be a huge shock to both the characters and the readers. Naruto has always been seen as a pure - hearted hero of the Leaf, and Deidara was a dangerous member of the Akatsuki. Naruto's character development could take a different turn. Maybe he would have a different view on the Akatsuki and their motives. He might even try to convert Deidara to the side of good, which would be a very interesting story arc. And his friends in the Leaf might be very suspicious of him at first, but Naruto would surely prove his loyalty in his own unique way.
In this fanfiction, perhaps it's a story of redemption for Deidara. Naruto, as his brother, could be the one to show him a different path. They might go on adventures together, facing various challenges that test their relationship. It could also explore how their powers might be related or different, given their connection as brothers.
If Naruto was Deidara's brother, their relationship would be a study in contrasts. Naruto, with his bright and sunny personality, would be the polar opposite of Deidara's brooding and destructive nature. At first, Deidara might resent Naruto for being so different from him and for having a more positive view of the world. But as time passed, Naruto's unwavering belief in the good of people could start to rub off on Deidara. They might have some intense arguments about their different worldviews, but also share some quiet moments where they just understand each other as brothers. And Naruto would always be there to pick Deidara up when he was down, even if Deidara didn't always show his appreciation.
In a fanfiction, Deidara and Sasori could be sent on a secret mission to infiltrate a hidden village. Deidara uses his clay creations to distract the guards while Sasori sneaks in with his puppets. As they progress, they discover a powerful weapon that the village is developing. They then have to decide whether to destroy it or take it for the Akatsuki. Along the way, they also have to deal with their own internal conflicts and the pressure of the mission.
Naruto and Deidara are the key characters. However, characters like Tobi could also be important. Tobi has his own relationship with Deidara in the original series. In this fanfiction, he might react differently to the news of Naruto being Deidara's brother. And from Konoha, Jiraiya could be a character in the story. He could be a mentor figure to Naruto, guiding him on how to handle this new family situation while also being aware of the danger that Deidara poses.
In some fanfictions, Deidara might give Naruto training on explosive jutsu control. Since Deidara is known for his explosive clay techniques, he could potentially teach Naruto how to better handle and utilize explosive elements in combat.
They could be on a journey to find a rare material for Deidara's art. Sasori, with his knowledge of the ninja world, helps guide them. Along the way, they encounter various strange creatures and ninja clans. They might have to fight off some who are after the same material.
There's a fanfic called 'Deidara's Redemption in Naruto's World'. In this story, Deidara has a change of heart after multiple encounters with Naruto. It delves deep into how Naruto's unwavering belief in the good of others starts to affect Deidara and his view of the ninja world.
One idea could be that they discover they are brothers in a hidden past plot twist. Maybe their parents were from different villages but fell in love secretly. As brothers, they could team up to face a new and powerful enemy that neither could defeat alone.