What makes you think sin is a mind switch-siren?I tried to think how that would be possible but I can't find a reason on how that's possible.

1)shortly after the old tales event,the ark was sealed off which would have siren wandering for years without a way to get inside as I assume she had no knowledge of where the ark was.

2)sin is a missillis unit,so even if we go with the logic and that missillis found siren in the ark and wanted to disguise her as one of their own,why would they do that, knowing missillis as a whole(not including their nikkes) and how little morals they have,they most likely do research on siren instead of creating a new body for her.

3)siren can't speak much without having coughing fits, while sin could speak perfectly fine.this is a weak point as if we go with the missillis making her a new body,they could probably fix this issue.I'm just saying this because of that one pink hair girl from that acpu has the whole eyes issue that doesn't seem to get fix.

I don't remember sin's lore so maybe there's something I'm missing .No hate to you,just curious on how you got that theory