Twin Sibling (3.5e Trait)
From D&D Wiki
You have a twin, one that you have lived with your entire life.
Benefit: +2 Dodge bonus to AC when standing with your twin; if twins are within 5 feet of each other they gain a further +2 Dodge bonus against flanking. If twin is identical, you get +15 disguise bonus to pretend to be the other twin. If wearing the same thing, this changes to +20.
Drawback: If not fighting alongside twin, DC 15 Will Save (Every time twin is dealt damage by an attack) if failed must use move action to move towards twin. If twin is incapacitated and in immediate danger of death, you must go help twin (unless pass a DC 25 will save). If Twin is Identical, you may suffer -5 to diplomacy skills while your twin is in the room, because people may find their presence disconcerting.
Special: Character must have a twin (if triplets everything stacks, and saves go up by 5). All twins have this trait by default, it only comes into play when both are in combat. A twin loses this trait if they spend a long time apart(Years, or decades.) but can regain it after the same time is spent with twin.
Roleplaying Ideas: Polar opposites that are still twins, identical fighters, shield wall of twins (If Triplets or Quintuplets... or more). Kobolds get this trait a lot in massive clutches.