In this story, two neighbors—as close as siblings—spend their summers in a makeshift wooden fortress. As they grow older, the "No Girls Allowed" signs are replaced by whispered dreams of the future. The conflict arises when they realize that their bond has outgrown the treehouse, shifting from platonic loyalty to a protective, blossoming romance.

Books in this category range from sweet to extremely steamy. Popular titles include:

Because of the sensitive nature of the keyword, digital platforms handle this content with strict moderation. Major self-publishing retailers like Amazon Kindle Direct Publishing (KDP) have rigid policies regarding themes of incest, often banning books that depict biological sibling romances.

Stories featuring step-siblings or adopted siblings who are not biologically related. This is a massive sub-genre in contemporary romance (often called "Step-brother romance").