you have to remember that Sephiroth was pretty much human and Jenova never knew his limits so thats why he seemed so weak throughout the game you battle Jenova's body but when you see Sephiroth you get the idea of divinity how can I explain that? who cares aslong as i do it

Sephiroth was the most powerfull human at his time as shown in clouds flashback but we dont know if it was true as zack was there instead of cloud and the memory could have been screwed up, Jenova on the other hand has alot of likitations because there are people with power who can destroy her, Jenova was partly alive before the reunion if she wasnt than Sephiroth wouldnt be in the materia and the game would be screwed so what now?

we know that jenova is alien but how do we know that only the planet has lifestream maybe Jenova's home had something similar to it who knows but Jenova couldnt absorb the lifestream for some reason so when she woke up she used the strongest and most vulnrable creature on the planet which was at the time Sephiroth in his what do we say? mid life crisis? well while he was all confused his mind was open so Jenova came in took it over bye bye Sephiroth and FF7 storyline begins