Name of novel: Undercover agent posing as heroine's long lost brother...?

It's a mystery/romance novel. The heroine is some sort of fashion designer (or similar profession), and she's found a long lost brother or half brother. She is going to meet him and instead steps into the middle of an investigation. The man she meets and thinks is her brother is actually an undercover agent (FBI maybe) and he lets her believe that he is her brother. But then they are attracted to each other and so on....When she finds out that he's not her brother, he still has to make her stay at the house because of the investigation....That is all I can remember. Help! Thanks in advance!