Hmmm, so would it be possible to use a construction like "слушок, что не кто йной, как племянник..." instead?
Holy cow! Is your native language another Slavic one? Or have you been living in Russia for many years now?
P.S. Oh, wait... I guess I misunderstood! You were saying that you're not a native speaker of English, and weren't 100% sure about whether the English construction sounded colloquial? In any case, "it was no less than" or "it was none other than" both sound totally okay to me.