Why is Koro-sensei's sibling Itona not like him? When I watch the anime I get excited by Koro-Sensei's new student, Itona. He has many similarities with koro-sensei. So, my question is: why do they look different? Koro-sensei is like an octopus and Itona is like a human.
As @Toshinou Kyouko explained, Itona and Korosensei aren't actually related.

Koro-sensei was...


   ... originally the subject of human experimentation. His powers are the result of the experiments that were run on his own body.


Itona is considered being Koro-sensei's brother because...


   ... he received his tentacles from the same scientists who experimented on Koro-sensei. His powers are inherited from Koro-sensei.




As for why they don't look alike:


   Itona had his tentacles implanted into him. Those tentacles were most likely originally part of Koro-sensei's body after the latter turned into the octopus we now know and "love". Unlike Koro-sensei, his body didn't actually change.