Truly madly guilty by Liane Moriarty

What did happen at the barbaque?
The barbaque that Clementine, Sam and their children, with their friends Erica and Oliver went to at their neighbors Vid and Tiffanys. All of the “responsible” adults have secrets that become difficult to cope and live with after the barbaque.
A compelling book that keeps you guessing till the very end.

Also by Liane Moriarty
Three wishes (audio book)