What makes parents and children into a family is not the fact that they love each other, or that those parents begot those particular children, but that they understand and keep their obligations toward each other.
Remember that the Ten Commandments do not say, "Love your father and mother."
Parents may want love very much; some of them may demand it, and even make the lives of the children miserable if they think that they do not get enough of it.
However, love is neither commanded nor demanded; rather, the commandment is (Exodus 20:12), "Honor your father and your mother."
Love within the family is a very good thing, but the commandment is to honor—or, in other words, to keep the formal and informal obligations of children toward parents.
That is true about siblings as well.
Far beyond the love or the biological connection, it is the mutual obligation between them that comprises the core family tie.
I want to stress this point, because we are living in a time, and within a culture, that often substitutes love for obligation.
Through the influence of romantic literature, among other reasons, we tend to think that love makes the family.
Love may make the family a glorious place to be, but it is keeping the rules—whatever these may be, and they do change from one culture to another, and from one family to another—that creates the family.
–Rabbi Adin Steinsaltz
From Simple Words, p. 182, by Rabbi Adin Steinsaltz