Correction on that statement. Sepiroth wasn't too much different. Everyone including him were drawn to the Jenova reunion (therefore, every living organism that was injected with Jenova cells was being summoned/controlled). The evidence supporting that is also shown in FF7 AC. Cloud's allucinations were given by Jenova's residue cells. The Children were also under Jenova's control after drinking Kadaj's black water (which was nothing more than Jenova cell infected water). Jenova's reunion is what controls Cloud as well as Sepiroth, who only gains full control of the situation around the end of the first FF7.