Objection 1. It would seem that we ought not to love more those who are more closely united to us by ties of blood. For it is written (Proverbs 18:24): "A man amiable in society, shall be more friendly than a brother." Again, Valerius Maximus says (Fact. et Dict. Memor. iv 7): "The ties of friendship are most strong and in no way yield to the ties of blood." Moreover it is quite certain and undeniable, that as to the latter, the lot of birth is fortuitous, whereas we contract the former by an untrammelled will, and a solid pledge. Therefore we ought not to love more than others those who are united to us by ties of blood.
Objection 2. Further, Ambrose says (De Officiis i, 7): "I love not less you whom I have begotten in the Gospel, than if I had begotten you in wedlock, for nature is no more eager to love than grace." Surely we ought to love those whom we expect to be with us for ever more than those who will be with us only in this world. Therefore we should not love our kindred more than those who are otherwise connected with us.
Objection 3. Further, "Love is proved by deeds," as Gregory states (Hom. in Evang. xxx). Now we are bound to do acts of love to others than our kindred: thus in the army a man must obey his officer rather than his father. Therefore we are not bound to love our kindred most of all.
On the contrary, The commandments of the decalogue contain a special precept about the honor due to our parents (Exodus 20:12). Therefore we ought to love more specially those who are united to us by ties of blood.
I answer that, As stated above (Article 7), we ought out of charity to love those who are more closely united to us more, both because our love for them is more intense, and because there are more reasons for loving them. Now intensity of love arises from the union of lover and beloved: and therefore we should measure the love of different persons according to the different kinds of union, so that a man is more loved in matters touching that particular union in respect of which he is loved. And, again, in comparing love to love we should compare one union with another. Accordingly we must say that friendship among blood relations is based upon their connection by natural origin, the friendship of fellow-citizens on their civic fellowship, and the friendship of those who are fighting side by side on the comradeship of battle. Wherefore in matters pertaining to nature we should love our kindred most, in matters concerning relations between citizens, we should prefer our fellow-citizens, and on the battlefield our fellow-soldiers. Hence the Philosopher says (Ethic. ix, 2) that "it is our duty to render to each class of people such respect as is natural and appropriate. This is in fact the principle upon which we seem to act, for we invite our relations to a wedding... It would seem to be a special duty to afford our parents the means of living... and to honor them."
The same applies to other kinds of friendship.
If however we compare union with union, it is evident that the union arising from natural origin is prior to, and more stable than, all others, because it is something affecting the very substance, whereas other unions supervene and may cease altogether. Therefore the friendship of kindred is more stable, while other friendships may be stronger in respect of that which is proper to each of them.
Reply to Objection 1. In as much as the friendship of comrades originates through their own choice, love of this kind takes precedence of the love of kindred in matters where we are free to do as we choose, for instance in matters of action. Yet the friendship of kindred is more stable, since it is more natural, and preponderates over others in matters touching nature: consequently we are more beholden to them in the providing of necessaries.
Reply to Objection 2. Ambrose is speaking of love with regard to favors respecting the fellowship of grace, namely, moral instruction. For in this matter, a man ought to provide for his spiritual children whom he has begotten spiritually, more than for the sons of his body, whom he is bound to support in bodily sustenance.
Reply to Objection 3. The fact that in the battle a man obeys his officer rather than his father proves, that he loves his father less, not simply but relatively, i.e. as regards the love which is based on fellowship in battle.
Source: Summa Theologica (New Advent)