This tag is to be used if the protagonist has a sibling who is not related by blood. These siblings are expected to either be in a romantic relationship, or show affection towards each other which goes beyond the expected intimacy of siblings. These siblings knowing that they are not related by blood is recommended, though them finding it out later i the story is also accepted. However, at some point of time in the story, they should find this out. This tag is not to be confused with Incest.