Its said in one of the videos with Gast, talking about he origin of JENOVA. She mentions that it came in the form of dead mothers, dead brothers.
Also, we have no reason to believe that the Sephiroth clones can shapeshift, as Yazoo, Laz, and Kadaj are not Sephiroth clones at all. And we do know what at least one Sephiroth clone looks like: Cloud. He looks like himself. There is nothing to suggest that any of the others look like anything other than what they looked like before being experimented on.
Also, we have no reason to believe that the Sephiroth clones can shapeshift, as Yazoo, Laz, and Kadaj are not Sephiroth clones at all. And we do know what at least one Sephiroth clone looks like: Cloud. He looks like himself. There is nothing to suggest that any of the others look like anything other than what they looked like before being experimented on.
Actually, we know three clones - Cloud, Zack, and the guy in the slums who "are sick".
Ok, anyway, I'm more or less ready to be converted.
I still think this presents issues with the narrative - the biggest one being Masamune. The second biggest being that Sephiroth is needlessly given physical form in North Crater despite the fact that he doesn't need it in any way shape or form to complete his plan.
The third issue has to do with the fact that Jenova somehow reverts back to her form in battles for no apparent reason.
The fourth being that you would think Sephiroth would realize that Jenova wasn't an ancient, if he was in total control of her mind.
I think it's safe to say that these are inconsistencies brought fourth by differnt writers and producers not really knowing where they were going with the narrative, and not enough coordinated efforts toward reconciling various scenes in the plot that were developed with large gaps in time, or with different team set-ups.
It's a common problem in FF games, but I think it sorta damages the claim that FF7 is easily followable as a stand-alone title.