How Nezuko Is Immune To Sunlight

Tamayo's analysis of her blood shows Nezuko’s blood has blue elements to it, and since the Blue Spider Lily has the power to cure Muzan of his weakness to sunlight, the anime could be hinting that Nezuko’s immunity comes from the Blue Spider Lily.

Blue Spider Lily bloomed near the Kamado family’s home, with Tanjiro being shown its location by his mother. 

Tanjiro was the only kid who saw the flowers open, but that doesn't mean his brothers and sisters never saw them.

 It's possible that Nezuko could have seen and eaten the Blue Spider Lily, which would eventually result in her becoming immune to sunlight as a demon.