Well, before they can fight Sasuke has to find Itachi.
I don't know how he's gonna do that trick.
In a normal fight one on one Sasuke could never beat Itachi right now.
Sasuke can't even beat Kakashi yet, even if he doesn't use MS.
So Sasuke killing Itachi would either make Itachi look weak or Sasuke way too strong.
"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"