Matter By Iain M. Banks

In a world renowned within a galaxy full of wonders a crime within a war. For one brother it means a desperate flight and a search for the one – maybe two – people who could clear his name. For his brother it means a life lived under constant threat of treachery and murder. And for their sister it means returning to a place she’d thought abandoned forever.
Only the sister is not what she once was; Djan Seriy Anaplian has become an agent of the Culture’s Special Circumstances section charged with high-level interference in civilisations throughout the greater galaxy.
Concealing her new identity – and her particular set of abilities – might be a dangerous strategy. In the world to which Anaplian returns nothing is quite as it seems; and determining the appropriate level of interference in someone else’s war is never a simple matter.