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.

https://screenrant.com/how-star-lord-mantis-siblings-mcu/