Sonia and I have a lot in common. Our parents are divorced. Our dads are gay. We both love barbecue potato chips. But she is different from me in at least one way: you can't tell how she's feeling just by looking at her. At all. When Bea's dad and his wonderful partner, Jess, decide to marry, it looks as if Bea's biggest wish is coming true: she's finally (finally!) going to have a sister. They're both ten.

They're both in fifth grade. Though they've never met, Bea knows that she and Sonia will be perfect sisters. Just like sisters anywhere, Bea thinks. But as the wedding day approaches, Bea makes discoveries that lead her to a possibly disastrous choice.