Sisters and brothers can be a little like friends and a little like enemies. But once you have a sibling, you are stuck with them, for better or worse. Two new teen novels explore this reality.

What if it was absolutely impossible to live with a crazy, threatening brother? In "Riding Invisible" (Hyperion, $15.99, 12 and up) by Sandra Alonzo, with illustrations by Nathan Huang, that's the reality that Yancy Aparicio faces.

Yancy is always with his horse, Shy, and always trying to avoid his brother, Will. Will is a monster.

Will has Conduct Disorder, a vaguely defined mental illness. In Yancy's eyes, Will's illness manifests itself in acting sweet to his parents and evil to other people.

Yancy long ago gave up trying to report Will's bad behavior. Telling resulted in zero discipline for Will and 100 percent payback for being a tattletale.

When Yancy refuses to give Will any of his money instead of giving in as usual, Will retaliates by cutting off Shy's tail. A follow-up attack on Shy -- a slash made by some scissors -- and a threat to "chop him into little pieces" sends Yancy on the road.

He imagines his Missing poster this way: "Half Latin male, 15 years old, 5'6", 140 lbs, riding horse in the photo." He starts out on the road toward Palmdale, a town about 50 miles from his house.

Unfortunately, his adventurous escape turns out to be more like a crash course in Homelessness with a Horse. Then Yancy is lucky enough to meet up with Tavo, an illegal immigrant who just happens to be the man in charge of the stables at Triple R Arabian Farm.

At this point it would be easy to assume that the plot will be predictable, resolve happily and end with the protagonist heading home a changed character. Alonzo sidesteps that expectation, sending Yancy home to deal with his problems face-to-face.

This detour is just one of several unexpected twists, resulting in a surprisingly funny and fresh narrative about how a family deals with mental illness.

The diary format and sketchy but expressive cartoon-style illustrations recall Jeff Kinney's very popular "Diary of a Wimpy Kid" series of books. The resemblance stops on the surface, however.

This is also a book for boys, but for an older audience. And Yancy is a very real, emotionally wounded adolescent boy, not a caricature.

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"Wish," the first novel from Alexandra Bullen, also tackles a weighty topic. Olivia is entering Golden Gate Prep right after spring break.

Her parents couldn't wait until the end of the school year to move to San Francisco -- not because of any urgency on the part of Olivia's mother's law firm, but because Violet, Olivia's twin sister, has just died.

Bullen doesn't reveal how Violet died until the book is nearly over, and Olivia doesn't need to dwell on the incident because Violet is magically returned to her.

Olivia's looking for a seamstress to repair a torn vintage Pucci dress of her sister for a party. She ends up in Mariposa of the Mission, an almost-empty storefront occupied by a girl named Posey.

Posey takes her dress but returns an entirely different one to Olivia's house, embroidered with a tiny butterfly. Unlike Cinderella, Olivia's night in the dress is not exactly enchanting.

At the spring solstice party, she chats up the green-eyed, skateboarding, rock band drummer that she's had her eye on. Back onstage, he motions into the crowd for her to come up and celebrate the solstice countdown. Except he's really motioning to his girlfriend, Callie, the hottest girl in school.

Olivia ends her night in the back of a cab, heading home while feeling utterly lonely. She makes one wish: to have Violet back. A glowing butterfly emerges from her dress and flies into the night.

Nothing else is out of the ordinary until the next morning, when Olivia awakens to the smell of cigarette smoke. It's coming from the tower room that would have been Violet's if she had lived.

Olivia enters it to find Violet perched on the windowsill. She didn't smoke when she was alive, but, as she tells Olivia, one of the perks about being a ghost is that cigarettes won't kill you.

They go back to Mariposa of the Mission and discover that Posey's dress was magic, and that Olivia gets two more wishes -- in the form of magical dresses -- from Posey. The only catch is that the wishes have to come from her heart.

Olivia has trouble thinking of other wishes. She has the only thing she really ever wanted: her sister. What more could she ask for?

Violet functions as Olivia's snarky, motivational ghost godmother. She nudges Olivia to talk to her crush's girlfriend, get new clothes, and generally come out of her shell.

Their relationship is at the heart of the novel. Violet and Olivia get along, but don't agree on everything. They lean on each other to de-stress but also know how to push each other's buttons. Of course, the situation is a little different now that Olivia is the only one who can see and communicate with Violet.

Ms. Bullen does a great job of spinning a fantasy that doesn't get bogged down in the mechanics of the magic at work. Instead, she gets right to the effects of the magic: the sweetness of having a sister back and the decisions that come next.

Violet's presence allows Olivia to start living a fuller life again, even though Violet is still dead. Ms. Bullen doesn't dwell on it, and her resolution to the problem is complex yet satisfying. Teens will feel the emotional impact without getting dragged down in a depression-fest.

"Wish" and "Riding Invisible" offer stories about contemporary teens and their siblings and use a light touch to illuminate dark situations. Both books will be enjoyed by thoughtful readers.

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