There could be. Fanfiction is all about taking existing characters and creating new scenarios. Given the popularity of both the Harry Potter universe (assuming Harry here is from that) and Superman, it's not a stretch to think that someone has written a story where they are brothers. However, it may not be a very common one as they come from very different fictional worlds originally.
It might also be a term of endearment for a younger male friend or acquaintance. In some groups, people use 'brother' in a friendly way, and 'little brother' could be a specific person within that group who is no longer interested in stories.
There's not enough information to say exactly who the little brother is. He could be an ordinary boy in a family who is the youngest among his siblings.
At first, there might be some misunderstandings. Naruto is used to the shinobi world and its ways, while Superman is all about truth, justice, and the American way. But as they spend more time together, they would start to respect each other's strengths. Naruto would admire Superman's raw power, and Superman would be impressed by Naruto's strategic thinking and his ability to come back from tough situations.
In the novel, Ye Xiu was a male. He didn't have a brother or a sister.
While waiting for the anime, you can also click on the link below to read the classic original work of " Full-time Expert "!
One possible plot could be that Harry discovers his true identity as Superman's brother. He might have been separated from Superman at birth for some reason, like a threat to their family. Then, as he grows up in the wizarding world, he starts to show some superhuman abilities which he doesn't understand at first. Eventually, through a series of events, he meets Superman and they realize their connection. Together, they use their combined powers of magic and super - strength to fight a common enemy that threatens both the wizarding world and Metropolis.
Well, one possibility is that Naruto's ninja skills combined with Superman's superpowers would make them an unbeatable duo. Naruto could use his shadow clones to create diversions while Superman uses his strength and flight to take on the main threats. They might also teach each other things, like Naruto showing Superman some stealth techniques and Superman teaching Naruto how to better control his super strength if he had any in this fanfic.