Example code that will successfully add a sibling Document from an onBeforeAdd on the Document ItemType using fake proeprties

docOnBeforeAdd.cs

Innovator inn = this.getInnovator();
if (this.getProperty("is_sibling") == "1") {
  return this;
} else {
  Item newDoc = inn.newItem("Document", "add");
  newDoc.setProperty("item_number", this.getProperty("item_number") + " - Design");
  newDoc.setProperty("name", this.getProperty("name") + "'s Sibling");
  newDoc.setProperty("is_sibling", "1");
  newDoc = newDoc.apply();
}
return this;