First of all, is your brother a person with full capacity for civil conduct? What do you mean? That is, a natural person whose elder brother is over 18 years old or over 16 years old but under 18 years old but whose main source of income is his own labor is a person with full capacity for civil conduct;

Secondly, did your brother get your authorization? Even if you don't get your authorization when signing, you can acknowledge the signature by ratification after signing. If you don't want to acknowledge this signature (provided that you don't authorize it to your brother), you can refuse to ratify it, but at this time your brother may bear civil liability (depending on whether the other party pursues it).