Yeah, SD isn't happening. I don't think that station will ever allow a show like SD in that slot again. Especially because Sana is caring, people apologize to each other, discuss their problems, and work things out with each other. Also, considering that there is absolutely no build up or negative changes in any of the character, no motive, and every character seems reasonable, it just seems impossible.
So, here's some actual speculation:
1. Sana attempted suicide
2. Nanaka's uncle is a hitman
3. The grandma hired Nanaka's uncle to burn her house down, because she got into a fight with her mother
4. Hinako is Nanaka's sister, sent away at an early age to be raised, and abandoned after the fire
5. Aoi has been hiding something very important this entire time
6. Syuri has been beaten ever since her mother died, just like her mother used to
7. Syuri's mother died from a beating
If any of these are correct, I'd be shocked. But hey, you never know. Especially since nobody except the producers and developers of the game don't know the plot.