RIN-NE/WMG

Rin-Ne is effectively an orphan.
Implicitly confirmed in chapter 4, when his grandmother shows up and offers to let him come and live with her. (No guess that she's the same woman that Sakura encountered in the incident ten years ago that gave Sakura the ability to see spirits.)
 * More or less confirmed by chapter 7. He lives on his own in a club building on the school grounds. What happened to his parents, though, is still unclear (read: an obvious Chekhov's Gun).
 * Chapter 29:

When Rin-Ne said that he'd go to hell for buying a new school uniform, he was dead serious.
As "sort of" a Shinigami he's subject to strict rules, as to how much he can charge and what he can spend his money on. Since his junior high uniform still fits and is in good shape, a new uniform is a luxury, which he is prohibited from buying.
 * Not out-and-out jossed, but chapter 7 makes it look a lot less likely. Rin-ne has incessant money problems because he has no family support and, as only part shinigami, he needs expensive devices to help him do the job of a shinigami. Which he has to do, to repay a debt his grandmother owes (part of the cost of her marrying his grandfather).
 * Once Rin-ne's dad is introduced, we learn that most of Rin-ne's money problems are because his father is such a skinflint (among his other faults) that he steals money from Rin-ne via embezzlement and identity-theft, anything to avoid paying for something himself.

Sakura is the real Yoiko Hibiki.
That would explain why Sakura's parents aren't shown, which is really weird in a Rumiko Takahashi series. That would also explain why she looks so much like Yoiko/Ranma-Chan, justifying Only Six Faces.

Sakura's family
Alternatively, Sakura could be Ranma's and Akane's child. It has been pointed out that Sakura looks like Ranma by the fans. She has been raised around such amount of craziness that she doesn't react to it, anymore. Being raised by her parents, she has also learned to avoid this behaivor. This could explain why her parents are nowhere to be seen (except for one dialog that didn't really give out much information, anyway). Rumiko Takahashi is saving The Reveal for later in the story.

Sakura will eventually get Shinigami powers and have to go save Rinne when he is taken to the Land of the Dead for breaking some sort of Shinigami rule or another.
As one translator already said, it's Rumiko Takahashi-style Anti-Bleach. Taking this theory to it's logical extreme, this is the only possible outcome.
 * The Shinigami powers part is unlikely, but the rest was dead on. Congrats.

It will eventually be made into an anime.
Because with the sucess of her past stories, it pretty much a given.

On how Sabato became such a Jerkass.
At first, he just wanted to help his mom. Due to her massive quota, he never got to see her much, as she was working very hard to pay it off. He figured that as a damashigami and illegally harvesting living souls, he could help her out. Unfortunately, down the road, he forgot what he was working for and became an asshole.

If Rinne is ever forced into controlling the Damashigami Corporation, he will proceed to embezzle.
He needs money, the damashigami are bad, and, quite frankly, it would be nice if he could get back at his father in some way.