>>42373705
Except he didn't say that, he said they are drawn to each other due to lack of mental illness. Something that more or less means RD actually should have had a chance too. The statement doesn't actually say much about romance endeavors either.

Back to the point, Flawless herself says 50/50 because she is. It's been explained more or once but Luna doesn't factor in DNA wise as her mind wasn't part of it. Luna put Trixie's mind into a box with a direct mental line to her, Rarity used a transfer spell to shove Twilight into that box. The box is not meant to hold two so instead of housing both or even letting Twilight in, it smashed Half of Twilight into half of Trixie resulting in Flawless who for the same reason only resembled and has parts of Twilight and Trixie.

Rarity and Luna are essentially just catalyst to push those two together to create only 1 Mare, their DNA and Minds aren't part of flawless as the birth is tied two Trixie and Twilight's minds touching there for they'd only be parents if you were to say the doctor and nurses who delivered your kids or the owner of a hotel were your family as well. That is the level of connection Luna and Rarity share to flawless, the openly the room door for Trixie and Twilight to mentally touch tips