One of the author's five top SFF books exploring sibling relationships, as shared at Tor.com:
False Hearts by Laura LamRead about another book on the list.
Conjoint twins Taema and Tila are the protagonists of Laura Lam’s near future thriller, False Hearts. The twins were physically separated at age sixteen, when they fled a cult that banned modern medicine in order to get medical assistance for their failing (single) heart. Now living apart from her sister and in very different worlds, Taema is suddenly thrust into a world of danger when Tila shows up on her doorstep, covered in blood and accused of murder.
The narrative is told in alternating perspectives each chapter from the twins, and their differences in nature are explored both in flashbacks to their youth, when secrets were impossible, and the modern day, where Tila has become involved in a very deadly underground world, and Taema must impersonate her sister to save her life. Their closeness and distance is a crucial part of the narrative as Taema yearns to understand who her sister has become, but also fears what she finds.
--Marshal Zeringue