Twins Ysabel and Justin feel like a bomb has been dropped on their family when they learn that their father is transgender.
The story is told in alternating chapters, with Ysabel and Justin taking turns narrating. Unfortunately their voices aren't terribly distinct, and you don't know why one has been given their bit of the story to tell over the other. While I found the sixteen year olds' resistance to their father's identity a little immature, their mother's total acceptance (after a bout of depression) implausible. The book has a didactic feel to it, like Davis wrote it on request, but without having a deep understanding of the issues herself. I could be totally wrong about Davis's transgender knowledge and experience; I'm just saying that's how it reads to me.
I'll also cop to the possibility that I wasn't crazy about Happy Families because it's more of a tween than a teen book, and I'm generally not a tween fiction fan.