Cross the Line (Blurred Lines #1)
Jenny Plumb

David and Nina are happily married, and in public they lead very conventional lives; he’s a veterinarian, and she’s a photographer. But in private, they are both Dominants who enjoy playing with a select few submissives, and Littles that they can top and ‘parent’ together.
Andie’s life changes drastically the day she accidentally hits a dog with a stolen car and meets David. Without quite knowing how—or why, Andie realizes that the kind older man and his wife are becoming more and more involved in all aspects of her life, including the legal trouble she's managed to get herself into.
Soon, the three of them are forming a platonic family unit without knowing each other’s secrets, but will mere friendship be enough for them, or do they all need something more... extraordinary?