The Rift — Nina Allan

Selena and Julie are sisters. As children they were closest companions, but as they grow towards maturity, a rift develops between them.
There are greater rifts, however. Julie goes missing at the age of seventeen. It will be twenty years before Selena sees her again. When Julie reappears, she tells Selena an incredible story about how she has spent time on another planet. Selena has an impossible choice to make: does she dismiss her sister as a damaged person, the victim of delusions, or believe her, and risk her own sanity in the process? Is Julie really who she says she is, and if she isn't, what does she have to gain by claiming her sister's identity?