i find it interesting that the same year shoghi started the IBC he declared his final brother a covenant breaker in Dec. 1951, of which he probably planned ahead for. It would be hard to get more info on that development. Before 1951 all other aghsans were declared covenant breakers if i remember correctly
Shoghis siblings:
Riaz, CB1951
Husayn (served as Shoghi's secretary) CBer 1945
Ruhangiz CBer 1940s (married into MA's family)
Mehrangiz CBER 1941 (married into MA's family)
It would make sense that after removing his final blood relative that he set up the IBC the way he did and when he did. Culturally a successor served as a secretary according to SE in several statements and SE was AB's secretary as well note the role of the IBC:
Between 1951 and 1957 Shoghi Effendi directed the members and used the Council to create an image of an international body handling the Bahá'í affairs in Haifa. According to Shoghi Effendi, the Council's responsibilities were to:
establish links with the Israeli authorities, and
negotiate with them concerning an establishing of a Bahá'í court to deal with personal matters,
assist Shoghi Effendi to complete the superstructure of the Shrine of the Báb
serve as an international Bahá'í secretariat.
http://en.wikipedia.org/wiki/International_Bah%C3%A1'%C3%AD_Council