Fan Casting Cain and Abel and Seth in Sort by Tropes

In this trope, a third person, mainly a Big Brother Bully or an Annoying Younger Sibling, is introduced specifically in a way to complicate the relationship between established siblings, or between the established siblings and the new sibling, hostile or not. If the characters aren't related by blood, but through the same Meta Origin consider themselves Blood Brothers or Bash Brothers literally, then another character with the same Meta Origin and honorary sibling status may suffice.