When one of my 2nd great-grandfathers' parents both died, he was taken in and raised by an aunt and uncle. He is attached to that family by a "Foster" relationship. If I run the Relationship Calculator or Relationship Chart functions between me and the descendants of the Foster parents, it shows the Foster parents as the common ancestors.
The Relationship Chart and Calculator functions should be changed so that following Foster and Adopted relationships are optional and not required. There are some times that you are interested in only the blood relationships.