Warning: the following contains spoilers for Guardians of the Galaxy Vol. 2.
A new fan theory suggests two Guardians of the Galaxy Vol. 2 heroes are brother and sister. Possibly the most intriguing characters introduced in the sequel is Mantis, a somewhat mysterious alien played by Pom Klementieff. When we're introduced to Mantis, she is acting as a sort of walking battery recharger for Ego (Kurt Russell), using her empathic abilities to sooth his hyper-active mind so he can sleep. Mantis soon develops a weird bond with Drax, who appears to like her even though he continually refers to the ugliness of her appearance (she remains oblivious to the insults, which makes her come across incredibly naive and incredibly pure).
It isn't really emphasized in the film, but Mantis does also have something of a connection to Peter Quill aka Star-Lord. Peter, of course, is the biological son of Ego and has been seeking his father all across the galaxy, while Mantis is basically a daughter to Ego having been found by him as a larva and kept in his company because she's useful to him. The connection is a bit tenuous as presented, but in a sense Peter and Mantis are brother and sister (it lends an extra note of emotion to the fact that they team up to kill Ego in the end). But what if the connection between Mantis and Peter is actually more meaningful than the movie portrays?
A Reddit user named JMH1982 has advanced the theory to suggest that Mantis and Peter actually are blood siblings. The theory goes that when Mantis explains that she was found by Ego as a larva on her home planet, she doesn't know that her mother was actually one of the many women impregnated by Ego as part of his massive plan for galactic domination, the same plan that resulted in the birth of Peter.
JMH1982's theory further states that Mantis doesn't know that she is Ego's biological daughter because he's been lying all along and that he kept her alive only because of her empathic abilities. He goes on to speculate that the brother-sister connection between Peter and Mantis will come into play at some point in the future.
The theory sounds valid on the surface, when you consider that family is such a huge thematic thread running through Guardians of the Galaxy. For Peter to lose a father (actually two fathers, when you consider what happened to Yondu) but gain a sister would make a certain kind of sense. Given the obvious story parallels between Guardians and Star Wars, it would definitely not be a shock for Mantis and Peter to eventually discover their sibling relationship in a way that echoes the Luke and Leia reveal from Return of the Jedi (let's hope Mantis and Peter don't complicate this reveal by doing anything they shouldn't in the meantime).
That said, there's a sense in which such a move might actually be missing the point. By the end of the film we've learned the important lesson that blood doesn't necessarily make a family, a point that's driven home when Peter's biological father Ego turns out to be a raving jerk while his adoptive father Yondu becomes a hero willing to sacrifice his own life. Indeed, Mantis and Peter don't have to be blood siblings in order to develop a brother-sister bond. Peter is already brothers with all his fellow Guardians, and those relationships are already as complex and sometimes frustrating as can be.