It's been speculated, even in the family tree, that Melody the Kidnapped had given birth to a child during her captivity. I not only suggest that this is true, but that the identity of the baby is actually Hesperia the Singer of Dusk, whom Solaria took in, claimed was her own daughter, and raised her in order to preserve her beloved sister's final legacy...which beggars the question of what happened to the baby Solaria had been pregnant with. Now, again, the easiest explanation is Solaria's baby was stillborn, but my theory is Solaria gave birth to a son. And she knew as well as everyone else that a male heir to the throne would be unacceptable to the people, and she had no desire to see her son develop an inferiority complex or be superseded by Hesperia or any potential daughters Solaria had (which is ironic, considering canon), so she used the dimensional scissors and turned her son into a Doorstop Baby on Earth...who happened to land on the doorstep of Angie's ancestor. So that's how Marco can claim descent from the Butterfly family as well: as the descendant of the son of Solaria the Monster Carver, while Star is a descendant of the daughter of Melody the Kidnapped. (It is also possible that something happened during her giving birth that made it clear to Solaria that she'd never have more children, hence her need to ensure Hesperia got the throne despite being technically a bastard.)
- This might even explain why Hesperia and Aurora are anomalies in the 'star-day-night queen' prophecy. If Solaria did have a surviving child (either the theorized boy or a girl she believed stillborn but somehow ended up on Earth), then it's a branch line on the throne that should not be. (And if Melody never married Hesperia's father, it's an illegitimate line, to boot.) Hesperia is the Dusk Queen because she is not part of the 'true' line of the succession, but the false one, hence the 'dusk' of the true Queens of Mewni. And Aurora is the Dawn Queen because she's where the true line (Marco) and the false line (Star) are united and the rightful rulers are restored, hence the 'dawn' of the renewed Queens of Mewni.