Mapping the Bones by Jane Yolen

The year is 1942, and Chaim and Gittel, Polish twins, are forced from their beautiful home and made to live in the Lodz Ghetto. Their family’s cramped quarters are awful, but when even those dire circumstances become too dangerous, their parents decide to make for the nearby Lagiewniki Forest, where partisan fighters are trying to shepherd Jews to freedom in Russia. The partisans take Chaim and Gittel, with promises that their parents will catch up — but soon, everything goes wrong. Their small band of fighters is caught and killed. Chaim, Gittel, and their two friends are left alive, only to be sent off to Sobanek concentration camp.

Chaim is quiet, a poet, and the twins often communicate through wordless exchanges of shared looks and their own invented sign language. But when they reach Sobanek, with its squalid conditions, rampant disease, and a building with a belching chimney that everyone is scared to so much as look at, the bond between Chaim and Gittel, once a source of strength, becomes a burden. For there is a doctor there looking to experiment on twins, and what he has in store for them is a horror they dare not imagine.