We introduce Corsican Twin, a tool for authoring augmented reality data visualisations in virtual reality using digital twins. The system provides users with the contextual information necessary to design embedded and situated data visualisations in a safe and convenient remote setting. We created system via a co-design process which involved people with little or no programming experience. Using the system, we illustrate three potential use cases for situated visualizations in the context of building maintenance, including: (1) on-site equipment debugging and diagnosis; (2) remote incident playback; and (3) operations simulations for future buildings. From feedback gathered during formative evaluations of our prototype tool with domain experts, we discuss implications, opportunities, and challenges for future in situ visualisation design tools.