How to get related nodes for node reference?
I don't know of an API function specifically for that but it's fairly simple to implement one using existing API methods:

function get_related_nodes($master_nid, $field_name) {
  // Load the 'master' node
  $master_node = node_load($master_nid);

  // Get the node reference items attached to this node
  $items = field_get_items('node', $node, $field_name);

  // Loop through and grab the nids
  $nids = array();
  foreach ($items as $item) {
    $nids[] = $item['nid'];
  }

  // Return the fully loaded reference nodes
  return node_load_multiple($nids);
}