In fanfiction, their bond is shown through Sonic's constant support for Tails. He'll praise Tails for his inventions, which makes Tails feel valued. For instance, when Tails creates a new gadget, Sonic is the first to test it out and give positive feedback.
Fanfiction sometimes shows their bond in more tender moments. Like when Tails is feeling down or homesick, Sonic will be there to cheer him up. He might tell Tails stories of their past adventures or simply sit with him in silence, which is a very brotherly way of showing care and understanding.
The brotherly bonds are also portrayed by their shared experiences. They face numerous challenges together, like fighting against new villains in the Sonic universe. Through these battles, they learn to rely on each other more and more. Sonic trusts Tails' intelligence to come up with strategies, and Tails trusts Sonic's speed and strength to execute them.