So what's with the imouto turning into another person? What's the symbolization of that?
So far the characters has been fulfilling some kind of rule, that is their Adolescence Syndrome anomalies are based on popular hearsay:

Sakuta: The boy that cried wolf
He tried to preach to people about the Syndrome, but people won't believe it until the end, even if himself contacted it. Also symbolized by the claw mark.

Mai: Rabbits die when they are lonely
As she disappeared from people's mind, she'll cease to exist. Also she's wearing bunnysuit.

Tomoe: Insanity is doing the same thing over and over and expecting a different result.
Not to quote Far Cry, but it's the closest.

Rio: There's at least three person with same face Quite literal, but there's one more: If you met with your doppleganger, you will die.
To 'die' in meaning of 'ceasing to be the old you', since after meeting her other self, albeit through phone, she became more honest about her feelings.

Now, for Kaede, she got two different ones too. Care to guess, non-novel reader?