Book Found: Looking for book with boss and assistant

I am looking for a book where the heroine is either the assistant to hero or his brother. She believes she is in love with his brother and hero knows it. When heroes brother announces his engagement to heroes ex, hero ask heroine to help him break them up. Hero wasn’t in love with ow or looking to get her back he just knows she using his brother and that his brother just wants what he had. I remember a scene where all four are at a brunch with hero and brothers mother, heroes brother starts paying attention to heroine and hero doesn’t like that. That’s all I can remember. Thanks for any help you can give me.