Help the concert-goer ask their friend to confirm what they are seeing.

"Look at the stage! _____ your brother playing the drums for the opening band?"

The correct answer is Isn't that.

Negative questions are frequently used when we expect the listener to agree with us or confirm something we think is true.

Since we are asking about "that" (a singular subject pointing to the person on stage), we use the singular verb Is. In a contracted negative question, it becomes Isn't that. "Is not that" has incorrect word order; the uncontracted form would be "Is that not".