Plain Code

$.each({
    prevOf: "previousSibling",
    nextOf: "nextSibling"
}, function( method, traversal ) {
    $.fn[ method ] = function( selector ) {
        return this.pushStack( this.map(function() {
            var ret = this[ traversal ];
            while ( ret && !$( ret ).is( selector ) ) {
                ret = ret[ traversal ];
            }
            return ret;
        }) );
    };
});