Objection 1. It would seem that we are nor bound to do good to those rather who are more closely united to us. For it is written (Luke 14:12): "When thou makest a dinner or a supper, call not thy friends, nor thy brethren, nor thy kinsmen." Now these are the most closely united to us. Therefore we are not bound to do good to those rather who are more closely united to us, but preferably to strangers and to those who are in want: hence the text goes on: "But, when thou makest a feast, call the poor, the maimed," etc.
Objection 2. Further, to help another in the battle is an act of very great goodness. But a soldier on the battlefield is bound to help a fellow-soldier who is a stranger rather than a kinsman who is a foe. Therefore in doing acts of kindness we are not bound to give the preference to those who are most closely united to us.
Objection 3. Further, we should pay what is due before conferring gratuitous favors. But it is a man's duty to be good to those who have been good to him. Therefore we ought to do good to our benefactors rather than to those who are closely united to us.
Objection 4. Further, a man ought to love his parents more than his children, as stated above (Question 26, Article 9). Yet a man ought to be more beneficent to his children, since "neither ought the children to lay up for the parents," according to 2 Corinthians 12:14. Therefore we are not bound to be more beneficent to those who are more closely united to us.
On the contrary, Augustine says (De Doctr. Christ. i, 28): "Since one cannot do good to all, we ought to consider those chiefly who by reason of place, time or any other circumstance, by a kind of chance are more closely united to us."
I answer that, Grace and virtue imitate the order of nature, which is established by Divine wisdom. Now the order of nature is such that every natural agent pours forth its activity first and most of all on the things which are nearest to it: thus fire heats most what is next to it. In like manner God pours forth the gifts of His goodness first and most plentifully on the substances which are nearest to Him, as Dionysius declares (Coel. Hier. vii). But the bestowal of benefits is an act of charity towards others. Therefore we ought to be most beneficent towards those who are most closely connected with us.
Now one man's connection with another may be measured in reference to the various matters in which men are engaged together; (thus the intercourse of kinsmen is in natural matters, that of fellow-citizens is in civic matters, that of the faithful is in spiritual matters, and so forth): and various benefits should be conferred in various ways according to these various connections, because we ought in preference to bestow on each one such benefits as pertain to the matter in which, speaking simply, he is most closely connected with us. And yet this may vary according to the various requirements of time, place, or matter in hand: because in certain cases one ought, for instance, to succor a stranger, in extreme necessity, rather than one's own father, if he is not in such urgent need.
Reply to Objection 1. Our Lord did not absolutely forbid us to invite our friends and kinsmen to eat with us, but to invite them so that they may invite us in return, since that would be an act not of charity but of cupidity. The case may occur, however, that one ought rather to invite strangers, on account of their greater want. For it must be understood that, other things being equal, one ought to succor those rather who are most closely connected with us. And if of two, one be more closely connected, and the other in greater want, it is not possible to decide, by any general rule, which of them we ought to help rather than the other, since there are various degrees of want as well as of connection: and the matter requires the judgment of a prudent man.
Reply to Objection 2. The common good of many is more Godlike than the good of an individual. Wherefore it is a virtuous action for a man to endanger even his own life, either for the spiritual or for the temporal common good of his country. Since therefore men engage together in warlike acts in order to safeguard the common weal, the soldier who with this in view succors his comrade, succors him not as a private individual, but with a view to the welfare of his country as a whole: wherefore it is not a matter for wonder if a stranger be preferred to one who is a blood relation.
Reply to Objection 3. A thing may be due in two ways. There is one which should be reckoned, not among the goods of the debtor, but rather as belonging to the person to whom it is due: for instance, a man may have another's goods, whether in money or in kind, either because he has stolen them, or because he has received them on loan or in deposit or in some other way. In this case a man ought to pay what he owes, rather than benefit his connections out of it, unless perchance the case be so urgent that it would be lawful for him to take another's property in order to relieve the one who is in need. Yet, again, this would not apply if the creditor were in equal distress: in which case, however, the claims on either side would have to be weighed with regard to such other conditions as a prudent man would take into consideration, because, on account of the different particular cases, as the Philosopher states (Ethic. ix, 2), it is impossible to lay down a general rule.
The other kind of due is one which is reckoned among the goods of the debtor and not of the creditor; for instance, a thing may be due, not because justice requires it, but on account of a certain moral equity, as in the case of benefits received gratis. Now no benefactor confers a benefit equal to that which a man receives from his parents: wherefore in paying back benefits received, we should give the first place to our parents before all others, unless, on the other side, there be such weightier motives, as need or some other circumstance, for instance the common good of the Church or state. On other cases we must take to account the connection and the benefit received; and here again no general rule can laid down.
Reply to Objection 4. Parents are like superiors, and so a parent's love tends to conferring benefits, while the children's love tends to honor their parents. Nevertheless in a case of extreme urgency it would be lawful to abandon one's children rather than one's parents, to abandon whom it is by no means lawful, on account of the obligation we lie under towards them for the benefits we have received from them, as the Philosopher states (Ethic. viii, 14).
Source: Summa Theologica (New Advent)