The Spirit House — William Sleator

A foreign exchange student comes to live with Julie's family for a year -- setting off a frightening chain of events  

Julie isn't thrilled about her parents' decision to sponsor a foreign exchange student from Thailand. Not only is the stranger going to live with them for a year, but Julie is convinced he'll be a nerd and will embarrass her at school. But when a tall, handsome, super-cool guy named Bia arrives, Julie is suddenly the envy of all her girlfriends.
 
Dominic, Julie's 11-year-old brother, is also thrilled to have Bia around the house. So to make the guest feel at home in America, Dominic builds a traditional Thai spirit house in the backyard. Overnight, Bia seems to undergo a major personality change. He's mean and spiteful and lies about everything. He also seems terrified of something . . . or someone. Has Dominic's construction somehow invoked a vengeful spirit? Is Bia the bearer of luck so bad it could harm Julie and her family?