The organization teaches that the heavenly aspect of the organization is the mother of the anointed and God is their father but that Jesus is the Father of the other sheep making God their Grandfather. Though as the bride of Christ, the anointed should be considered the mother of the other sheep and the WTS does not consider the other sheep "sons" of God.
As to the 1957 WT quote, a careful check of the WT-CD and reading their comments in context was to "mother," I found that each mention was about God's wife, the heavenly organization, the "mother" of Christ.
*** it-2 p. 694 Prophecy ***Places were used prophetically, the city of Jerusalem on Mount Zion at times being used to represent a heavenly organization that is the “mother” of spirit-anointed Christians. (Ga 4:26)
*** it-2 p. 1198 Woman ***Abraham’s wife Sarah, Paul says, corresponds to “the Jerusalem above,” who is the spiritual mother of Paul and his spirit-begotten associates. This heavenly “mother” would be also the “mother” of Christ, who is the oldest among his spiritual brothers, all of whom spring from God as their Father.—Heb 2:11, 12; see FREE WOMAN.
*** w57 5/1 p. 275 par. 5 Show Respect for Jehovah’s Organization ***In this way Paul identifies Jehovah’s wife as His invisible universal organization, the mother of Christ and of his joint heirs.
* w95 8/1 p. 13 par. 19 Jehovah—A God Who Teaches ***Even though the other sheep are not directly included among the “sons” spoken of at Isaiah 54:13, they are blessed with being taught by Jehovah. Therefore, they properly address God as “Father” because he will, in effect, be their Grandfather through the “Eternal Father,” Jesus Christ.—Matthew 6:9; Isaiah 9:6.
*** w96 8/15 p. 21 Do You Remember? ***Who is the Serpent and who is “the woman” referred to at Genesis 3:15?The Serpent is not the lowly snake but the one who used it, Satan the Devil. (Revelation 12:9) “The woman” is not Eve but Jehovah’s heavenly organization, the mother of his spirit-anointed servants on earth
*** w56 10/1 pp. 604-605 par. 24 ‘He That Marries Not Does Better’ ***Thus Jehovah’s purpose in beautifying this earth with human marriage will be gloriously accomplished. By Jesus Christ the Everlasting Father, Jehovah God will give eternal proof that marriage was certain to be a complete success on earth. Marriage will have worked in full harmony with his purpose in creating the earth and putting perfect man upon it, namely, to fill an earthly paradise with a perfect, righteous human family, in full vindication of his bringing in the irreproachable arrangement of marriage. Not only will eternally saved mankind on the paradise earth rejoice evermore but also the great heavenly Husband Jehovah and his faithful, beloved wife, his universal organization in heaven under Jesus Christ. Jehovah as an affectionate Grand Father and his universal organization as a tenderhearted Grand Mother will forever join in expressing their loving-kindness to their grandchildren, their God-fearing offspring on earth.