This is a charade - two parts are joined together to form the answer. Look for how the clue can be split into components that combine.
The word 'souper' isn't standard English - it's a play on words. Think of what 'soup' is as a noun, then consider how '-er' might be added to create someone who does something with soup.
Answer: BROTHER | BROTH (soup) + ER (one who does something) = BROTH-ER, a play on 'souper' meaning someone who makes or serves soup, giving us a male relative