The Vanishing Half by Brit Bennett

 Desiree and Stella are twin sisters living in tiny Mallard, Louisiana during the 1960s. Mallard is unique in that a large part of its black population is very light skinned, which proves to be a crucial factor in the story. The sisters decide there is more to life than living in Mallard, so they take off for New Orleans, where Desiree finds a job at a laundry but Stella ends up with a job as a secretary--where they think she is white. She kind of likes living this secret life and decides to stick with it, moving to Boston with her boss when he gets a new job (without notifying Desiree) and eventually becoming romantically involved with him, without ever telling him her true story. Desiree, in the meantime, marries a black man, whom she eventually flees with her very black daughter, Jude. The story alternates between Desiree--who has never stopped trying to find her sister, Stella--who has never told a soul that she is really black, and Jude--who accidentally stumbles upon a link to Stella, then isn't sure what to do about it.

It's a fascinating story of how different the two sisters' lives end up being--and the consequences of the decisions they made as teens.