Unknown Book Title

[Kendall]

Hello,
I’m looking for a romance where the hero is rich and has a married younger sister. The heroine is the nanny to the younger sister’s two boys. The married sister believes the heroine and her husband are having an affair (they are not) so she calls the hero to do something about it.
The hero eventually marries the heroine to keep her away from his sister’s husband. The heroine finds out and wants to break things off.

I can’t remember the title of this story, please help!!!