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: When not fighting alongside twin, if the twin is in danger from an enemy you get a -2 to hit any enemy except the one endangering the Twin. When separated from the twin or you two end up in a fight (for one reason or another) the two become shaken for the battle+1d3 hours or until twin returns. If twin is incapacitated and in immediate danger of death, you must go help twin (unless pass a DC 25 will save). If the twin has died, upon their death, you must make a will save DC 15 + Wisdom Modifier or go insane for 1D4 years.
Special: Character must have a twin (if triplets everything stacks, and saves go up by 5). All twins have this trait by default. If you two are separated for too long or have a conflict for a long while (months or decade. DM's discretion), you may remove the trait.
Roleplaying Ideas: You and your twin tend to get along with each other, even if you are polar opposites. You two feel uncomfortable fighting each other and prefer to fight side-to-side and doing so tends to confuse the enemy.