The spelling of "co sister" can be a bit tricky due to its unique construction. This term refers to the sister of one's spouse. The word is spelled with a space between "co" and "sister," and is pronounced [koh-sis-tur] using IPA phonetic transcription. The "co" prefix in this context indicates that the person is related through marriage, rather than blood. It's important to remember to include the space in the spelling of "co sister" to differentiate it from the word "cosister," which means "to grow together."