It looks like Mantis was intended to be Star-Lord's sister - at least in the development of Guardians of the Galaxy Vol. 2. Director James Gunn recently confirmed that another of Mantis' species appeared in Guardians 2 as one of Ego's many, many love interests, but he seemed to err away from suggesting this was Mantis' mother (and thus making Mantis Ego's daughter and Peter Quill her half-brother).
In the released Guardians of the Galaxy Vol. 2, Mantis' backstory is rather simple: she was found by Ego when just a larva and was raised on his planet, kept around to allow the Celestial being to sleep. She witnessed his repeated attempts to find an heir powerful enough to help him take over the universe, eventually turning on her master-turned-captor with the Guardians.
However, it may have originally been more complex. Jennifer Sharp, the actress who played the bug alien whom Ego romanced, posted on Instagram (via Reddit) a photo of her on the set of Guardians of the Galaxy Vol. 2 captioned "It was an honor to work with Kurt Russell even for a short flashback scene in Guardians of the Galaxy Vol 2 as Mantis' Mom(even though the scene was cut)." Since Gunn's confirmation of the Easter egg and dismissal of Mantis' relation to Ego, however, that's been edited to remove any mention of a flashback scene or her relationship to Mantis (and also correct the grammar on the shortening of "Volume").
It's entirely possible that Sharp was simply assuming her character's parentage of Mantis based on them being the same species - indeed, as the role was presumably a single take (Gunn later said she was a model for the CGI of Ego's flings) then she may not have been given full info of the character. This would make the earlier post inaccurate and the edit a correction.
However, given how tight the idea that Mantis is Ego's offspring - and was kept around only because of her ability to help him rest - it's possible that this was once the plan in the making of Guardians of the Galaxy Vol. 2 but was then pulled back to instead be ambiguous or otherwise a mystery. The only word prior to this week was Jennifer Sharp saying she was Mantis' mother, and Gunn hasn't explicitly said she isn't, just that it shouldn't necessarily be inferred - something the edits to Sharp's Instagram also suggest. As the detail is so slight, it could have never actually have meant to be confirmed.
As for why things are being avoided now, it may be done to allow a retcon of the fact down the line, but could also be to hide a Guardians of the Galaxy Vol. 3 twist. If Mantis is Star-Lord's half-sister, then that could have some pretty serious ramifications for the self-defined family team and their third adventure.
Before that third adventure, though, the Guardians need to come back: all but Rocket were turned to ash by Thanos' finger snap at the end of Avengers: Infinity War, although most of the cast - including Pom Klementieff as Mantis - are confirmed to be back for Avengers 4 and Guardians of the Galaxy Vol. 3.
- Ant-Man & The Wasp (2018) release date: Jul 06, 2018
- Captain Marvel (2019) release date: Mar 08, 2019
- The Avengers 4 / Avengers: Endgame (2019) release date: Apr 26, 2019
- Spider-Man: Far From Home (2019) release date: Jul 05, 2019