Sibling adoption is exactly as it sounds; It's the adoption of a "sibling set" for the sake of not splitting the children up to be adopted by separate families. Adoptable siblings are difficult to find homes for because they take more work, are more expensive to raise and will need their own space in the adoptive household. For those reasons, the demographics of a sibling adopter are unsuprisingly those who are older, have already raised children and want to experience of rearing children all over again without the high maintenance of raising the child from infancy. In an effort to encourage sibling adoption, some states may provide health and dental insurance for these children, if adopted.
Sibling adoption has the added benefit for older children by making the transition into a new lifestyle with someone else they can talk and relate to automatically. Siblings are also comforted by having a biological relative in the house with them.
International agencies, particularly those in China, are notorious for asking adopters if they want to adopt multiple children. Some are siblings and others are children around the same age that have been paired together. In some ways, adopting multiple children is beneficial because traveling overseas to adopt is expensive, and this saves an adopter a second trip if he or she wants to bring another child into the family later. It also gives the children someone else to relate to while they acclimated to the culture and any social barriers they may meet along the way.