also see silsilah
titles of endearment based on familial terms seem to be common in the letters of the Baha'i authority figures. For instance Shoghi signs his letters "your true brother" to Mason. Yet, he does this with several close Baha'is, Baha'is he had a close personal connection to.
Letter to Marzieh Gail:
Dear and valued co-worker:
Your many and valuable services, past and present, should encourage you in your efforts for the promotion of our beloved Faith. The friends, far and near, appreciate the value of these services, and you should not feel depressed or discouraged. I will continue to pray for you from all my heart. Persevere, be happy and confident.
Your true brother,
Shoghi
http://bahai-library.com/shoghieffendi_letter_marzieh_gail
10/24/1952
see also Members of the Holy Family