Since we have a break for a while (no chapt this week), i thought i would put out a lame theory (based on complete conjecture) i have had for a while, if nothing else, so we have something to post on.
As we know the Uchiha and the Senju used to fight at the behest of waring nations. The Senju possesing the 'Body' and the 'will' of the
younger brother, and the Uchiha possessed the 'eye' and chakra, from the the older brother, the two brothers decending from the Sennin.
When the Senju and the Uchiha ended this feud and joined together to create the leaf village, I think that during that reconciliation, a senju bride was taken by the Uchiha as a sign of faith and good will.
It would be doubtful that the 'pure' branch of the Uchiha would accept one of their own to marry in this fashion, so i believe it was a lesser branch family this bride was taken, not by the main branch, but a slightly lesser branch.
This lesser branch gave birth to Sasuke and Itachi's grand mother. She went on to have a girl (S&I mother).
As the village forgot the past Itachi and Sasukes dad married the 1/4 senju-Uchiha hybrid. Giving birth to Itachi, who inherited an affitity for the portion of the Senju body. Making him a sort of hybrid, with the best of both worlds. This ability to use this small portion of the Younger brothers 'power' in conjunction with his Sharingan (genjutsu), made not only his unrivaled power (his increadible Taijutsu) but also an ability to excel at Ninjutsu.
But upon awakening his MS his curse was complete. As he used his MS abilities, the small portion of Senju blood in him began to die.
Using his MS gradually weakened Itachi, It was poisoning him - He had a death sentence - leading to his eventual Suicide/ Death in the fight with Sasuke.
Sasuke did not inherit this portion of Senju blood, or he did, but he had a greater affinity for the older brother 'eye' and Chakra. Everyone who met Sasuke has said his eyes were greater than itachi's, but Itachi would still whoop his ass, Sauske even mentioned it in his first fight "what is this gulf between us?" It was because of the gift/curse of the Senju body.
It could also explain why Danzo and Co. were able to use Itachi as a double agent. They used his history of Senju blood against him.