For a long time they accepted single sisters as missionaries, as a fact, we lived with one in our missionary-home, from fifth class.
she died in 2009, I read.
This was under Knorr. After the seventies policy changed and they preferred single brothers or Couples.
To tell you the truth, it makes sense. As a home- overseer like I used to be it is like having an extra wife. If you want or not you feel responsible for the safety and health for such a sister.