Words matter. When we use the common vernacular and refer to a child as "natural" versus adopted, it implies something is wrong or false about adoption. In fact, there is nothing unnatural about adopting a child or being adopted. A more accurate way to refer to a child or family would be "birth" or "adoptive."
The effect on birth children of adopting a child depends on the parents' attitudes, on the age and emotional health of the children and their resilience. The placement of a newborn in typical health is mostly about adding a sibling. Adoption will add some complexity, however. The extent of this complexity will depend on the type of aodption as well as the dispositions of all involved. A transracially-placed child brings the entire family under more social scrutiny. Theatrical birth children love the drama; shy birth children don't like the added attention.
Adopting an older child adds another layer of potential complexity. This child comes into the family with his own life experiences. And, by definition, these experiences were suboptimal or else he wouldn't be in need of an alternative family. His history will impact everyone in his new family.
Some birth children develop wisdom and compassion as a result of adopting a sibling. But if, for instance, their parents have over-extended themselves in the attempt to rescue an orphan, then it's possible that all of the children may feel resentment at the lack of attention to their own needs.