I think most background checks reveal if you have a criminal background, financial issues, etc. I can't imagine very many (other than the very highest Pentagon, NSA, CIA, etc.) take a look at who your relatives are. And, even if they did a cursory check, if your sibling is a foreign citizen, there's not likely to be a paper trail connecting you to him, so it would be nearly impossible for a city government to discover that much information.
I'd still like to know if they even bothered with a cursory check.