your immediate family are your kids. You and they are entitled to ID90. You can nominate a traveling companion - which can be one of either: wife, parents, siblings. There are travel class (economy etc) limitations depending on whether it is domestic or International, and minimum time limits for you being in the company before these come into effect.
The second tier is ID50 which you can nominate your parents and siblings.
There are a few rules etc, but I think that is about the basic info