The animal model does not eliminate those half sisters, but, if only daughters of one bull appear in a management group, those daughters do not contribute to that sire's evaluation. An important new feature of the animal model is the use of all related animals in each animal's evaluation. The process might seem quite complex when first imagined, but the application is really quite simple. Each animal's evaluation affects evaluations of all relatives by first affecting evaluations on parents and offspring. The influence of a cow's PTA on relatives is like a ripple on a farm pond, ever wider in its influence, but less dramatic in impact as more and more remote ancestors and descendants are affected by her evaluation.
Unknowns contribute something to the animal model, however.
By forming groups of such animals and keeping track of genetic merit of descendants from these groups, the animal model can make a fairly reasonable guess about the genetic merit of the unknowns themselves. These guesses, properly weighted, are used in evaluating each animal further downstream. The actual impact of all these relationships on any animal's PTA under the animal model depends on other information available. Remote ancestors contribute little to evaluations, especially relative to information such as a large group of daughters on an AI sire. The animal model properly weights each item of information relative to other indicators of merit on each cow or bull evaluated.