If you decided to check out the Drax and Mantis episodes of Marvel Studios: Legends on Disney+ (which have since been removed), you'll have noticed the latter's recap featured a Guardians of the Galaxy Vol. 2 deleted scene. In that, Mantis told Drax that she's Peter Quill's sister, confirming a long-standing fan theory about Ego being her father as well.
In The Guardians of the Galaxy Holiday Special, the conversation between the two is repeated when Mantis opens up about her big secret with Drax (he already knows, so we're assuming that deleted scene is now canon).
Scared to tell Star-Lord the truth about their relationship in fear of reminding her teammate what happened to his mother, Mantis heads to Earth and kidnaps Kevin Bacon. We'll let you watch the phenomenal Special Presentation in its entirety to see how that all plays out, but in the final few moments, Mantis finally tells Peter the truth.
In a tearful exchange between the heroes, Star-Lord tells Mantis that the news she's his sister is "the greatest Christmas gift I could ever get." They hug, wishing each other a Merry Christmas and setting the stage for this new dynamic to be explored in Guardians of the Galaxy Vol. 3.
There are other big developments in this Holiday Special, including the revelation that the Guardians have bought Knowhere from The Collector (meaning he's still alive after falling afoul of Thanos in Avengers: Infinity War).
Gamora only gets a brief mention here, but there's a new addition to the team in the form of Cosmo. Maria Bakalova does a great job as the character, and her dynamic with Rocket looks set to be a lot of fun. Similar to her comic book counterpart, the space dog has telekinetic powers.
The Guardians of the Galaxy Holiday Special is now streaming on Disney+!