On my game at least 1.5 vanilla I can chose to control or not regardless of there being a family member in the reinforcement army.

If i wish to outnumber the enemy for a particular tactical advantage I always composite the second army without family ( coz they always seem to commit suicide charges ) and let them divide the enemy.
Or I control them, and use them as reinforcements by retreating archers when they are out of ammo etc.

I think that Celt was right in this, that if you check information when you return to the screen your options are back to standard and you have to re-check them.

Good luck.