The Losers at the Center of the Galaxy — Mary Winn Heider

A tuba player without a tuba and his jellyfish-imitating sister cope with their father's disappearance in this hilarious and moving novel by the author of The Mortification of Fovea Munson. 

When Lenny Volpe, former quarterback of the worst professional football team in the nation, leaves his family and disappears, the Chicago Horribles win their first game in a long time. Fans are thrilled. The world seems to go back to normal. Except for the Volpe kids.

Winston throws himself into playing the tuba, and Louise starts secret experiments to find a cure for brain injuries, and they're each fine, just fine, coping in their own way. That is, until the investigation of some eccentric teacher behavior and the discovery of a real live bear paraded as the Horribles' new mascot make it clear that things are very much Not Fine. The siblings may just need each other, after all.