When Guardians of the Galaxy Vol. 3 hits theaters we may learn that Mantis and Star-Lord are actually siblings.
Mantis and Star-Lord could very well be brother and sister as evidenced by a new Guardians of the Galaxy 3 theory.
Mantis was introduced in Guardians of the Galaxy Vol. 2 as a servant to Ego and then became a part of the team.
It seems highly probable that Ego is the reason that Mantis has such amazing powers like Star-Lord and we may learn the truth in Guardians of the Galaxy Vol. 3.