Dr. Calvin Zabo a/k/a Mister Hyde and his daughter Daisy Johnson a/k/a Quake
They never said that Skye is an alien, it's only a hypothesis of Coulson and May to explain why the transfusion isn't affecting her like him and Garrett. I highly doubt they're going to waste their premiere female superhero (Captain Marvel) on Skye and Agents of SHIELD.
You know, if THIS episode gets low raitings there are just plain HATERS out there