I think Hizashi became branch family because they were twins.
If Hizashi was two years younger he'd be in the head family too-
Maybe they did it just for the purpose we've seen in the flashback.
So that the branch family member can be sacrifed for the head family member if necessary.
But who knows
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"