Cockatiels can breed as early as 5-6 months old, if not younger... not that it should be encouraged as they may be too young, physically and mentally speaking, to be raising chicks... just saying it's not impossible
If the hen was not an Ino mutation, then yes, it would be possible to sex *some* of the chicks as soon as they hatch. However, since she is an ino, you wont be able to sex any of the chicks *unless* the male is split for cinnamon and/or pearl.
In other words, you'l need to sex based on behavior as they mature, or, as mentioned, DNA sexing.