I suppose we may know soon enough what is going on but here are some interesting panels.
First panel Kabuto is claiming its "his eye", theres only one genjutsu strong enough to do this.
Second panel Kabuto finds out that the eye actually belongs to Shisui and its his Koto amatsukami.
There was also the panels where Danzo was using Shisuis eye at the kages meeting and AO and the Mizukage believe that it was that type of power related to the previous Mizukage being manipulated.
Here is another panel in which Madara and Kabuto are referring to this person that Kabuto believed the eye to belong to with a genjutsu strong enough to control the Mizukage.
Here are 2 panels of Danzo using Shisuis eye, Shisui Doryuku is used but its not clear what exactly was done. It appears like it could be some type of space time ninjutsu.
We have only seen the mangekeyo awake in brothers up to this point so its possible Shisui is indeed Obitos brother. We will find out more soon enough but Shisui has some relation to the one pulling all the strings.