Re: Advant Children
***SPOILERS***
Marlene at the start made it out that it was the planets punishment, but later explained that it is jenova that is the geostigma. when people die, they return to the lifestream with the stigma (jenova), who then rots it and gains control, so sephiroth, and i quote "can use this planet as a vessel to travel the cosmos". kadaj is in it because cloud killed sephiroth, so he had to drink jenovas head to transform into sephy (not sure y as they r remnant of jenovas legs), also if the other 2 were to drink it they would also turn into sephy, thet would be a big problem. they call cloud a brother, because they were all hojo's experiments.
hope i cleard some stuff up there