Autobiography, read sometime between ’88-’92. As young children the author and her sister were placed in an orphanage by their mother. She describes her experience having TB. She did not realize until years later that it was an orphanage, I think she thought it was boarding school.
Could this be the autobiography Long Ago When I Was Young by E. (Edith) Nesbit? Edith suffered a great deal in and out of a number of different boarding schools and she had a sister, Mary, who had tuberculosis.
Solved by the original querent: the book is Orphans, real and imaginary by Eileen Simpson. It was her father who put her and her sister in the orphanage after their mother’s death. The first part is their story, then the second part is about orphans in mythology, fiction and factual accounts down through history.