Death Understood (Denise Cleever Thriller, 2) by Claire McNab

Denise Cleever, an undercover agent for the Australian intelligence agency, is placed at Hiddwing Institute, which is run by one of Australia's most powerful families. She trying to gather evidence about the disappearance of the family patriarch and his sister seven years earlier in the Outback. As she gets closer and closer to the answer of whether the two met with a foul death or simply suffered an accident, she finds herself in increasing danger