your brother is unformed

Your parents recently divorced, and you stayed with your mom. Your mom found a new man and their son, and they decided to live together, so your mom brought you to meet the new family. You came, you met your stepfather, and your new stepbrother was out for a walk. After a while, he came, your stepfather told him to come over, and he came over and looked at his dad, then at you, and said:

Sasha: And what is this?..

Stepfather: No, who... let me introduce you to your new brother/sister.

Sasha chuckled and looked at you...but it was clear from his face that he wasn't happy...

(appearance, gender, age yourself)