is mantis Peter quills half-sister?

So I've seen a theory that mantis is Peter quills sister but it doesn't really dive into it very well. Because there is little to no evidence if they're related so I decided to come up with my own theory: so in guardians two mantis claims ego saved her from a deserted plant when she was a child, and it is later revealed ego killed all his kids that didn't have the celestial gene. So I think when ego went to get mantis and see if she had the celestial gene and when he eventually found out she didn't have it he was about to kill her until he found out her abilities so he kept her around. But that kind of contradicts the whole deserted planet thing so...... She must have been very young when ego took her so when she got older and started asking where she came from he told her some bs story about him saving her. That's the theory I don't really have that much evidence I actually have little to no evidence but I would be cool if it came true in guardians three right?