Satou and Shio's names (meaning "sugar" and "salt" respectively), seem to be contrasting names. Shio's name especially may seem to not make much sense when the whole of the manga has Satou referring to her as making her own life "sweeter". However, salt is considered a flavor enhancer, and is often used in a number of recipes, including sweets and desserts, to enhance the flavors of them. This is exactly what Shio does for Satou. Brings out the sweet feelings in her that nothing and no one else has been able to.
Speaking of names, the two main male characters have paired names as well. Taiyou can mean "sun" while Asahi can mean "morning sunlight".
This association with the sun connects them with Shio as well. Shio is constantly associated with the symbol of "the moon" especially with "lunacy". The moon and the sun are often portrayed as opposites, however, the moon shines when touched by the light of the sun. Two of the people to whom she glows the brightest are people associated with the sun.
Many times in the series Satou has flashbacks to her childhood that involve her aunt teaching her about what is and isnt love. Then we find out just what kind ofperson heraunt is. We can only pray those lessons about "love" never got physical.
The finale of the anime leaves several disturbing implications. Shio is not only alive, but she either believes that she is Satou reborn or is developing mannerisms not too dissimilar from her. One can only ask if Shio would ever recover from her experiences, and if not, will she live Satou's life down to every detail climaxing in her kidnapping a pink-haired girl who she believed to be Satou's reincarnation?
As it turns out, even if they had managed to get on the plane, it's unlikely that Satou would've been believed dead as she'd hoped. Given that the police apparently identified Shouko's body as they announced her death on the news. Meaning that Satou's plan to fake her own death would have failed anyway as, since her body went undisturbed as is in the chair she was left in, there would've still been proof of the body being Shouko's. Even if her aunt had also tried to take the fall of murdering Shouko, there would be the question of where Satou was, given she was her legal guardian. Satou's plan to vanish off the face of the earth was doomed to failure no matter what.
Not necessarily. The police correctly identifying the body may have depended on the fact they had an extra body to account for. If Satou was the only missing person from the fire (also assuming Asashi didn't go to the apartment either), would the police simply close the case saying the body must be Satou?
Satou's Aunt may have fully admitted to burning down her apartment, but chances are she's not going to prison. Her psyche would ensure that the crime was a case of insanity rather than pure malevolence. Then the most likely outcome would be her going back to a "normal" life with a probation officer and/or regular psychiatric visits to the doctor. And unless they have strong enough wills, they too will fall under Auntie's spell...