"My brother is not bad."

Translation: मेरा भाई बुरा नहीं है।

[lotus.kirk]

Why can the 'nahin' not be before 'buraa' ?

[vinay92]

नहीं should always be placed adjacent to a verb. बुरा is an adjective so the only verb in this sentence is है which is why नहीं should be placed right before है.

[Owen-Benjambavan]

by adjacent you mean just to the left of the verb?

[vinay92]

It can also be to the right of verbs (except auxiliary verbs like है, रहा etc). Placing it to the right of the verb emphasises the negation. Eg: मैं सो नहीं रहा - I am NOT sleeping.