i know for a fact your immediate relatives: spouse, parents, children. you can also file for your siblings. we all know filing for siblings takes somewhere from 12-15 years in processing. if your siblings are single when you file them and they get married and have kids during the 12-15 years of waiting period, your siblings can add the additions to his/her family. so by the time the whole thing is processed, even if you only filed for your siblings, his/her whole family may be included by then.
but i also think if your siblings are already married and have kids, you can file for them as well. i'm just not sure if you have to do it all individually or per family.