We say "Blood runs thicker than water", but is there any philosophical tenability to this idea? If I feel my girlfriend's parents are mistreating her, should I step in, or should I keep my nose out of another family's internal business? Is it acceptable for me to prioritize non-family-members over members of my own family in cases where neither side clearly has greater need/right? Should children always turn to their parents before anyone else when they need help?

Rather a lot of questions there which have at their root the issue of whether we should take family as a relevant ethical issue. To say it is relevant is not to say that it always is stronger than any countervailing right. An adult who strikes a child may be in a morally stronger position if the child is his, but he is still not in a strong position, and others are entitled to intervene to protect the child, it seems to me. On the other hand, it would be going too far to insist that everyone is a perfect parent, whatever that might be, and to remove children from less perfect parents.

There are advantages in fostering strong family links since those who are normally closest to us might well know what is our best interests, and they might be more concerned for our welfare than comparative strangers. Not that this is inevitable of course, and when families break down there should be mechanisms to intervene. It is very useful for us to persuade ourselves though that we have a special responsibility for a child when it keeps us up all night and presents us frequently with unpleasant diapers, since otherwise it would be difficult to make sense of our caring behavior.

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