This is how it all went down. Jenova is not sephiroths mother, that lady luceria is, that lady that vincent talks to in disk two in that cave. Hojo is sephiroth's daddy which means those two did the dirty dirty. When he was born hojo experiemented on a bunch of people including sephiroth with the jenova cells. cloud was also part of the experiment when he so called joined SOLIDER. When Nibeleim burnt down, sephiroth found out he was part of a experiement and flipped out. He goes to mount nibel to free jenova, because he know feels she is his mother, maybe by manipulation, because it happens to could when he buggs out so it muts happen to him as well. Cloud ends up defeating sephiroth but obviously didnt kill him and five years later cloud ends up in midgar confused.