"Christ is really the Son of the Virgin Mother through the real relation of her motherhood to [Him]."

Bouguereau, Virgin and child (1888).

"each opinion is true to a certain extent. For if we consider the adequate causes of filiation, we must needs say that there are two filiations in respect of the twofold nativity. But if we consider the subject of filiation, which can only be the eternal suppositum, then no other than the eternal filiation in Christ is a real relation. Nevertheless, He has the relation of Son in regard to His Mother, because it is implied in the relation of motherhood to Christ. Thus God is called Lord by a relation which is implied in the real relation by which the creature is subject to God. And although lordship is not a real relation in God, yet is He really Lord through the real subjection of the creature to Him. In the same way Christ is really the Son of the Virgin Mother through the real relation of her motherhood to Christ."

"quantum ad aliquid utraque opinio verum dicit. Nam si attendamus ad perfectas rationes filiationis, oportet dicere duas filiationes, secundum dualitatem nativitatum. Si autem attendamus ad subiectum filiationis, quod non potest esse nisi suppositum aeternum, non potest in Christo esse realiter nisi filiatio aeterna. Dicitur tamen relative filius ad matrem relatione quae cointelligitur relationi maternitatis ad Christum. Sicut Deus dicitur dominus relatione quae cointelligitur reali relationi qua creatura subiicitur Deo. Et quamvis relatio dominii non sit realis in Deo, tamen realiter est dominus, ex reali subiectione creaturae ad ipsum. Et similiter Christus dicitur realiter filius virginis matris ex relatione reali maternitatis ad Christum."

     St. Thomas Aquinas, Summa theologiae III.35.5.Resp., trans. FEDP.  I was put on to this by John Saward, Cradle of redeeming love:  the theology of the Christmas mystery (San Francisco:  Ignatius Press, 2002), 126.