These days they air a commercial on TV for a book whose title escapes me, but which reportedly "is the story of three Jewish brothers during WWII".
In other words, it seems like the clause "during WWII" is meant to describe the "three Jewish brothers".
Usually, one would expect a prepositional phrase that is introduced by the preposition "during" to function as the adverbial of a sentence, as in: "We lived there during the summer" or "He didn't speak at all during dinner". The adverbial, then, is an complement of the sentence's main verb.
In "...the story of three Jewish brothers during WWII" however, there is no verb, and the prepositional phrase in cuestion would have to be interpreted as part of the noun phrase (NP) "three Jewish brothers during WWII".
But is it ok to use a "during"-phrase as the modifyer of a noun in the same way that we use other prepositional phrases, such as "from Germany" or "in bed"?:
a) "I helped three Jewish brothers from Germany"
b) "I helped three Jewish brothers during WWII"
When a prepositional phrase ("from Germany" / "during WWII") is part of a noun phrase, it cannot be separated from the noun ("brothers") which it modifies without altering the meaning of the sentence. So, sentence a) would actually become nonsensical, or at best odd, if we were to move the prep. phrase, (the original meaning would at any rate be changed): *"From Germany I helped three brothers"
Sentence b) would be acceptable, and it would retain the meaning that is most likely understood when one reads the original sentence (b)): "During WWII I helped three Jewish brothers". For sentence b), then, "during WWII" is automatically interpreted as a complement of the verb "helped", and NOT as part of the NP *"three jewish brothers during WWII".
The original text should probably have included a subordinate relative clause: "... the story of three Jewish brothers who lived during WWII"