The adoption of sibling groups is the adoption of multiple children who are biologically related to one another. This can refer to twins, or international "twin" adoptions, but more often refers to the public adoption of siblings.
In the event that siblings are removed from their home and put into foster care, the state tries its hardest to promote an adoption of the entire sibling group so as to not separate the children from one another. It's thought that sibling groups adopted into the same family adjust better to their new family than those that are separated or adopted individually. This is particularly important if the children experienced a traumatic event together. Their siblings are the only ones they may feel able to identify with.
The adoption of sibling groups are difficult to place, though. More children require more resources, financially, emotionally and time-wise. They also require more space in their adoptive home. Many parents may feel one child is an overwhelming change, so three may mean three-times the overwhelming change. Depending on the person, this may or may not be true. Older couples, particularly those who have raised biological children who have moved out, are more likely to adopt sibling groups. Often for the simple reason that they find raising a child from infancy to either be a tedious effort or unfair to the child who will have parents too far his or her senior.