All workers are half sisters because they all have the same queen mother.
But they don't all have the same drone father (queens mate with multiple drones).
Duh, I forgot about that. Somehow the bees know the geneology of the eggs?
Yes, they know their full-sisters from their half-sisters. There's a lot of politics that goes on between these sister groups when it comes to raising a new queen. There is also research that indicates that when a swarm is deciding on a new home, the sister groups may act as "voting blocs" for a location picked by a full sister versus a location picked by a half-sister.