BibTeX:
@incollection{AbdulBah1982,
  author = {'Abdu'l-Bahá},
  title = {O My Dear Sister!},
  booktitle = {Bahíyyih Khánum: The Greatest Holy Leaf},
  year = {1982},
  publisher = {Bahá'í World Centre},
  url = {https://www.gutenberg.org/ebooks/19242},
}

APA:
'Abdu'l-Bahá. (1982). O My Dear Sister!. In *Bahíyyih Khánum: The Greatest Holy Leaf*. Bahá'í World Centre.

MLA:
'Abdu'l-Bahá. "O My Dear Sister!." *Bahíyyih Khánum: The Greatest Holy Leaf*, Bahá'í World Centre, 1982.

Chicago:
'Abdu'l-Bahá. "O My Dear Sister!." In *Bahíyyih Khánum: The Greatest Holy Leaf*, Bahá'í World Centre, 1982.