I was just wondering- what are the exact rules about magic in front of Muggles, particularly say for a Muggle-born living with a Muggle/ with their families?

Because Harry in CS gets done for just being in the same house as a Hover Charm in front of the Durselys and, if I remember right, Vernon's boss. So if the boss hadn't been there would it have been all right? Are the Muggle family/ spouse of a wizard/ witch automatically exempt from the Statute of Secrecy? Which would explain why Hermione can perform a memory charm on her parents.

I'm asking because I'm writing a story about Justin Finch-Fletchely and his cousin. I want her to know about him being a Wizard. She lives with her aunt (his Mum) so would that mean she's allowed in on the secret? And also that he can't be punished for performing magic on her? Any help/ clarification would be great! Alex