Is Prejudice a Sin? Nope.

The question has been raised, so we took a look.

...According to the nature of the perfection which it limits, evil is metaphysical, physical, or moral. Metaphysical evil is not evil properly so called; it is but the negation of a greater good, or the limitation of finite beings by other finite beings. Physical evil deprives the subject affected by it of some natural good, and is adverse to the well-being of the subject, as pain and suffering. Moral evil is found only in intelligent beings; it deprives them of some moral good. Here we have to deal with moral evil only. This may be defined as a privation of conformity to right reason and to the law of God. Since the morality of a human act consists in its agreement or non-agreement with right reason and the eternal law, an act is good or evil in the moral order according as it involves this agreement or non-agreement. When the intelligent creature, knowing God and His law, deliberately refuses to obey, moral evil results....

(Found at "Nature of Sin" in the Catholic Encyclopedia)

The important word in the red-highlight area is "act."  Prejudice is not an "act"; it is more akin to a 'metaphysical evil' (above), it is a negation of a greater good.  Prejudice is not racism, which is the 'acting out' of prejudice; therefore racism is a sin, but prejudice is not.

Prejudice, however, is an evil, so we should resist it at all times.