1. The quote from Tobit is used because it is a good example of "brother" and "sister" being applied to a husband and wife in ancient Hebrew literature. It was used in the same way as the quote from the Penitential attributed to John the Faster: just to illustrate a point about how a word was used.
Christadelphians do not think the books of the Apocrypha (or the Penitentials) are inspired.
Come, you who are blessed by my Father; take your inheritance, the kingdom prepared for you since the creation of the world
Matthew 25:34, NIV