The accidental mother / Rowan Coleman.

Enhanced descriptions from Syndetics:

What do you do when you are a child' s last hope?

From bestselling author Rowan Coleman comes a deeply touching tale of a fast-track career woman whose life takes a sharp right turn when motherhood unexpectedly lands in her lap.

Sophie and Carrie were childhood best friends, but in the last few years they've lost touch. While Carrie chose motherhood in a small town, Sophie is powering up the London career track. She's a corporate manager poised for her next promotion. Sure, she doesn't have much time for men, but she has a great shoe collection and a cat who's never going to let her down.

And then Sophie is told that Carrie has died, with nobody left to care for her two daughters, Bella and Izzy, aged six and three. Their father, who left before Carrie's death, is nowhere to be found; their grandmother is moving into assisted living. Sophie once promised Carrie she would take care of her children if the worst ever happened...and now that day has come.

Witty, wise, and filled with genuinely powerful emotion, The Accidental Mother is the heartwarming, heartbreaking story of a woman who is woefully under-equipped to be suddenly thrown into motherhood, but who through the eyes of two little girls learns more about loss, commitment, and true love than she had ever realized existed.