We have a little sister, and she hath no breasts: what shall we do for our sister in the day when she shall be spoken for?
We have a little sister - the Gentile Church (Ezekiel 16:48-53). "We," i:e., the Hebrew Church, which heretofore admitted Gentiles to communion, only by becoming Judaic proselytes. Now first idolatrous Gentiles are admitted directly (Acts 11:17-26). Generally, the saint's anxiety for other souls (John 4:28-29).
No breasts - neither faith nor love as yet (note, Song of Solomon 4:5), which "come by hearing" of Him who first loved us. Not yet fit to be His bride and mother of a spiritual offspring.
What we shall do for our sister - the chief question in the early Church at the first council, (Acts 15:1-41.) How shall "the older brother" treat the "younger," already received by the Father? (Luke 15:25-32.) Generally the first question of every soul that has itself been brought to the obedience of the heavenly King (Galatians 6:10).
In the day when she shall be spoken for? - i:e., when she shall be sought in marriage (Judges 14:7), namely, by Jesus Christ, the heavenly Bridegroom.