However, Jenova is the cause for Sephiroth, thus Jenova is the true enemy. Sephiroth is the main villain and the strongest in the game, but that doesn't automatically make him the true enemy.
Jenova cells gave him the power he has, and made him the way he is. So in my opinion i think Jenova is the dark one behind the scenes pulling the strings
Expect everything that Jenova did was through Sephiroth's will. Sephiroth is the true villain of FF7 who is pulling Jenova's strings. Oh and by your logic Hojo would be the true villain because he is the father of Sephiroth and was the one who enfused him with Jenova cells.