NotFormer : Yet the GB, who are Jesus' brothers, refer to [the "other sheep"] as brothers and sisters. But how can that be?
This question reminded me that when the "other sheep" were first identified (they were also called Jonadabs) they were not included among Jehovah's witnesses, were not invited to the Memorial, and were addressed as friends or people of goodwill, not as brothers.
The "other sheep" were understood to be people in general who were devoted to righteousness, even though they were not dedicated to God or spirit-begotten, the ones of whom Rutherford spoke in 1918 when he said "millions now living may never die". In the 1935 Washington convention (to which Jonadabs were invited) they were identified with the "great crowd" of Revelation. In 1938 the Jonadabs were specifically invited to be present at the Memorial as companions of the remnant and thereafter became a greater part of the congregations.
There is a full discussion of this in the 15 February 1966 Watchtower article Identifying the Present-Day Beneficiaries [of the New Covenant].