In the Sonic franchise, including games, comics, and animated series, Knuckles and Tails are not depicted as Sonic's brothers. Instead, they are his friends and allies.
is Sonic's best friend and sidekick, a two-tailed fox known for his mechanical skills and flight abilities24.
is initially introduced as Sonic's rival but later becomes a friend. He is the guardian of the Master Emerald on Angel Island2.
In the live-action movies, there is no indication that Knuckles and Tails are Sonic's brothers. However, some fan interpretations or fan-made content might portray them as such, but this is not official canon1. In the movies, Tails and Knuckles do form close bonds with Sonic, but they are not depicted as siblings4.