Looking for a semi-post-apocalyptic book about a girl who is brought on as a helper for another girl being raised as a sacrifice

Question

There's a book I remember reading in the late 90's or early 2000's that I can't find the title of. The key points I remember are:

• A girl (Girl-A) is living in a sort of orphanage having been taken from the city. She reads through all of the generally available books in the system and is granted automated access to Shakespeare and the like.
• A family come to the orphanage looking for a helper for the Sacrifice they are raising. Girl-A is chosen because she has read so much.
• The sacrifice (Girl-B) is exceptionally sensitive to chemicals causing her to lose her hair, have peeling skin, and the like. She is being raised to take on a semi-religious role for the community in ritually testing the safety of food and water.
• Girl-A ends up doing the studying that Girl-B should be doing for her training and Girl-A continually provides the correct answers for Girl-B
• Girl-A has a cassette tape she has been carrying around for a long time. A mountain man character she was initially scared of (Name may have been Lem?) is able to build a cassette player to play it.
• Girl-A enjoys eating fish and this is considered expensive
• I believe one of the tests was set in the Alberta badlands but am not totally sure.

Anyone know this book?

Answer 1

The Secret Under My Skin

An excerpt from a review on Goodreads:

Blay Raytee was a street child scraping a living together on the streets of St Pearl with Hilary, her protector, until Hilary is taken by the Death Squad and Blay is rounded up by the Commission and sent to a workcamp in Kildevil. Like all the children in workcamps, she's given the name of the month she was taken: September, but in her heart she holds onto the only name she can remember, Blay Raytee. She doesn't know how old she is or where she came from, but when she's not scavenging the local landfill for recyclable objects with the other children, wearing heavy UV suits and visors, she's reading all the lastbooks and discovering ancient poets like Yeats.

There's a new bio-indicator in training, a girl called Marrella, living with the Master of the Way up the hill, who needs help with her lessons. The children who read the books at the workcamp are gathered and Marrella picks small, scrawny Blay. Life with the Master, William, and his wife Erica is the happiest time Blay's ever known, despite the mean, snobby Marrella who rejects Blay's offers of friendship. She also refuses to learn what she must to be bio-indicator, a sacrificial lamb of sorts dating from the time when the air was toxic and the water poisonous - now bio-indicators are more like religious symbols who, away from the control of the Commission, are helping to resurrect science. The planet, devastated by global warming and all that it entailed, has slowly recovered but the Commission has convinced people it's still dangerous to be out during daylight hours.

The cassette tape and the hermit, 'Lem Howl' are indeed there, as well as the need to create the player, and the device to scan the eponymous secret beneath Blay's skin.

Found with a search for book post-apocalyptic orphanage poison tester cassette