The Last Duke by Jess Michaels

When Christopher “Kit” Collins’ father dies, he is devastated, but he must swiftly take on the title of Duke of Kingsacre and all the responsibility that comes with it. Including the care of his very young illegitimate half-sister, who is now his ward. He is very upset when he discovers the governess his father hired for the child just before his death is Sarah Carlton.
Sarah is equally disturbed by the arrival of her new employer, as she and Kit once had an unpleasant encounter when she was at her lowest point. But since she has no other place to go, she can only hope he will not sack her. Thanks to the adoration of his sister, he does not and the two enter into a wary truce because they both care for the child.
A truce that explodes one passionate night after a nearly tragic accident. Comfort leads to more and soon the two are engaged in an affair. But when danger comes to threaten the child they both love, they must overcome their deeply rooted mistrust and work together to keep her safe. And perhaps find a way to make a family together.