In a pair of twins, only one has soul. It is known.
And I know exactly for which one of my sisters this applies.
I actually used that idea in an Arcana Evolved/Diamond Throne campaign.
There was a village were allegedly every generation or so, the villagers would kill a child. The Diamond Throne ordered one of its Commissars (an adventure with the license to conduct investigations n behalf of the Throne, securing support from local guardmens) to investigate the matter and put a stop to it.
Turned out that the reason the village did that was because every generation, a child would be born without a soul. In my mythology, a being without a soul has no moral compass, and can't tell right from wrong. Many forms of undead and constructs explicitely have no soul, making it easier for their creators to control them and do their bidding. Evil souls can tell right from wrong, they just don't care. Anyway, this would allow another entity to effectively take control of her.
An Oathsworn near the village had vowed once to stop evil entities from stealing the children for all eternity. He had died, long ago, never accomplishing his goal, so his Oath kept him in an undead existence. In addition, another Commissar had also arrived at the village. She was secretly working for a malovent entity (Virdella Tresham, from the setting book) from another material plane that wanted to capture the child so she would finally have a physical form. The entity's long term plan would destroy this plane to give her power to conquer many other planes. The Commissar hoped to rise in the ranks of her organization so she could find out her true plan and stop them, and tried to hide the child away. She justified her lan because the entity would be able to find a physical form eventually somewhere in the world, this way she would know what body, and gain the trust of her organization.
She used the fact that the child was actually a twin and had a ("soulful") sibling as a trick to get the investigation by the player Commissar and his party off track.
The party eventually managed to work with the Oathsworn against the second Commissar. They performed a ritual that basically allowed the soul of the Oathsworn to enter the child, and all future children that would suffer the same fate would equally gain his soul, so the problem would finally be resolved and no more children would need to be sacrificed.
