Sonic Underground isn't even a real game. Anyway, you spell the name like Manic. Real stupid once again. SONIC, SHADOW and SILVER are the brothers.
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No they are both different hedgehogs.
Silver the Hedgehog, Shadow the Hedgehog and Amy Rose the Hedgehog.
In the comic "Sonic Universe" Sonic the Hedgehog and Sally Acorn get married. They have kids, Manik and Sonia.
The Sonic franchise is somewhat infamous for being bloated with cheap, gimmicky characters and hedgehogs are among the most commonplace character species in the games, cartoons, and comics. From video games, the "true canon" of the series, we have Sonic, Shadow, Amy, Silver, Mephiles (technically the mind of Solaris, but appears as a hedgehog), and Metal Sonic (again debatable since he's technically a robot built by Robotnik and has several versions). From the cartoons, we were introduced to some more hedgehogs in the form of Sonic's family from Sonic Underground: Sonia, Manik, Uncle Chuck, and Queen Aleena. (It is also assumed that their father was a hedgehog, but he never actually appears nor is he mentioned). From the comics, we have a few more hedgehogs: Scourge or Evil Sonic, and Rob O'Hedge (Amy's cousin).
Sonic has a brother (Manic) and a sister (Sonia).