Father and uncle are distinguished by collaterality. The criterion of collaterality rests on the distinction between siblings and lineal relatives. In English, only cousin ignores the distinction of collaterality. Cousin also ignores the distinction made concerning sex of the relative concerned - the female child of our mother's brother is called by the same term as his male child. Our own kinship system ignores this - an uncle may be traced through either parent, a grandparent is a parent of either or our parents.
Polarity recognizes that a relationship consists of two parties, and thus two terms - aunt, niece.
If this criterion is ignored, the two parties call each other by the same name. In English, cousin is an example. Brother is not, although both brothers use the same term to indicate each other. Brother results from the fact that the same criteria is being used to assign both people to the classification. In English, we almost always acknowledge polarity.