Something that has always bugged me about Namine is the fact that she was born from Kairi's heart leaving Sora. It's clear that her appearance was influenced by Kairi's heart but Sora's body and soul were used in her creation. Isn't it odd that she returned to Kairi rather than Sora? 

It alllll comes back to the heart, man. 

Not really, she was born from Kairi's heart, but she didn't take Sora's body, she was just sort of born from it....I don't know, special Princess of Heart loopholes or something! 

It alllll comes back to the heart, man. 

^Basically this, plus Namine's symbolism in relation to Sora and Kairi is too much for me to handle

Well Roxas is supposed to be soras nobody aswell but he looks like Ventus because of vens heart inside sora. So I guess it really does all come back to the heart

And this, my friend, is why I see Namine as Roxas' sister.

Namine was created being the negative look of Kairi.

Light hair, light clothing, and is part of her because "Nami" means wave, where Kairi means sea.

She was created from Kairi's heart being released.

Nope, She was born from kairi's heart INSIDE Sora and now since Kairi got her heart back, Namine would go to her instead.