I am currently in the US on a h1b visa. My I-140 has been approved under eb3 category but looks like it will really take a while for the visa numbers to be current. My brother is a naturalized citizen. Can he file for my green card under family based siblings category? I was told by someone that i cannot file 2 immigrant petitions. Is that true?

You can simultaneously file through as many green card categories as you wish.