Star-Lord and Mantis are siblings and know they are related in Guardians of the Galaxy Vol. 3. Here is how they are brother and sister in the MCU.
Here is how Star-Lord and Mantis are siblings in the Marvel Cinematic Universe during Guardians of the Galaxy Vol. 3. The Guardians of the Galaxy franchise has always had a focus on family, specifically that of Peter Quill. The first movie revealed how Peter's journey to becoming Star-Lord began after he was kidnapped by Yondu shortly after his mother, Meredith Quill, died from cancer. While he did not know his father, Star-Lord first found a family with Yondu and the Ravagers. But, the formation of the Guardians of the Galaxy brought a new found family into his life thanks to the bond between him, Gamora, Drax, Rocket, and Groot.
It was not until Guardians of the Galaxy Vol. 2 that Mantis entered the franchise and became part of Star-Lord's life. She was revealed to be working with Ego the Living Planet, who is Peter's biological father. This marked the beginning of Mantis and Star-Lord developing a friendship as she joined the Guardians of the Galaxy to fight Ego. Her introduction brought theories that Mantis and Star-Lord were related, but the movie did not confirm it. Star-Lord and Mantis being siblings is known to all characters during Guardians of the Galaxy Vol. 3, as the MCU previously explained how this was possible.
Star-Lord & Mantis Were Both Fathered By Ego
Star-Lord and Mantis are siblings because they were both fathered by Ego. Star-Lord and Mantis have different mothers, but they both had romantic relationships with Ego in the past. This was the result of Ego's Expansion plan, which was his mission to terraform planets across the galaxy so they were controlled by him. He soon discovered that he alone was not powerful enough to fulfill this wish. That forced Ego to switch up his plan and begin having children with thousands of alien species with the hopes that some of them would inherit his Celestial powers.
Ego's relationship with Meredith Quill makes him Star-Lord's biological father. Guardians of the Galaxy Vol. 2 teased the various female aliens that Ego procreated with and the hundreds or thousands of his children that he killed upon realizing they did not have powers. His relations with a woman from Mantis' alien race resulted in the birth of the powerful empath. While Mantis is not known to have inherited any Celestial powers from her father, Ego still let her live so she could serve him. Since Ego is both of their fathers, that is how Star-Lord and Mantis are siblings in the MCU.
Star-Lord & Mantis' Relationship Was Revealed In The GOTG Holiday Special
The MCU confirmed Star-Lord and Mantis are related in The Guardians of the Galaxy Holiday Special. A deleted scene from Guardians of the Galaxy Vol. 2 was originally meant to confirm this fact as Mantis revealed it to Drax. Instead, James Gunn saved the twist and revealed it in The Guardians of the Galaxy Holiday Special. Mantis tells Star-Lord that she is his sister near the end of the Disney+ special as part of the explanation for why she wanted to kidnap Kevin Bacon for him. The confirmation of Star-Lord and Mantis'relationship allows Guardians of the Galaxy Vol. 3 to begin with it known to the characters.
