but silverJay could be right...remember how Bobito awakened MS when he saw Kakashi kill Rin??
Well I agreed with the statement somewhat, I only stated what was known in the DB. Though, the general understanding is that you have to lose someone close to you. Whether done by your hands, or through someone else. But the problem being a few factors...
I. Itachi awakened MS by seeing Shisui commit suicide
II. Obito awakened MS by seeing Kakashi kill Rin
Then you have an Uchiha like Sasuke who came into his home finding everyone dead, including his parents, and finding out that Itachi killed his family (AND reliving these moments in a Genjutsu), but still didn't activate MS.
He only activated it after 'witnessing' Itachi die, or sometime after. So the understanding is a immersive amount of negative trauma that is caused by losing someone close to you.
So it would be either; Sasuke, Sakura, and who ever would be enough to supply that amount of emotions. People tend to confuse Sharingan activation to MS activation. Emotional trauma activates the Sharingan, but what continues to evolve it is what Tobirama feared "Curse of Hatred". Though, not all of them fall into depravity. Shisui and Itachi being the number one showcases of that.