If I remember correctly the first part of the storyline when Sephiroth is condeming himself in the castle he pulls the weird robot thing {forgot it's name} that was guarding Jenova had a hologram of Sephiroth which we see later in the game and he {the holigram} wore a helmet that said Jenova so really Sephiroth got combined into one being with a conjuncting mind or it was Tifa in a very good cosplay costume. Also For the Sephiroth he thought he was his own mother I know that sounds crazy but maybes thats like a dream sequence except he thinks it's real and since he has such a strong spirit he could mentaly be at a diffrent dimension or he's really crazy.