Matthew Poole and continuators on Song of Solomon 8:8

8 We have a little sister b, and she hath no breasts : what shall we do for our sister in the day when she shall be spoken for ?

These are manifestly the words of the Bride, still continuing her speech. The present Church, which was that of the Jews, speaks of another future Church, which was to consist of the Gentiles, which she calls little, because she was the younger Sister, and then scarce had a being; and she calls her her Sister, partly because she was so in the purpose of God, their common Father, though at present she was a stranger to him; and partly to intimate that the Gentile-Church should be admitted to the participation of the same priviledges with that of the Jews.

No grown and full Breasts, as Virgins have when they are ripe for Marriage, Ezek. 16. 7. This signifies the present doleful estate of the Gentiles, which as yet were not grown up into a Church Estate, and wanted the milk or food of Life, as for itself, so also for its Members.

Teach us to know and perform our duty to them, which is to embrace them with sincere and fervent affections, to promote their coming in to Christ, and to rejoice in it, and not to envy it, and murmur at it, as the Jews did in the days of Christ, and of his Apostles.

To wit, for bringing her into the state of Matrimony: when Christ, and his Apostles, and others the first Ministers of the Gospel, who were Members of the Jewish Church, did speak and act for the conversion of the Gentiles.