In general, I'm pretty sure there's never more than one "queen". There's concubines, both legitimate and illegitimate ones, but only one "actual crowned woman" who sits beside the king as his lawful wife, and who has legitimate authority over the kingdom.
So I guess I'll have to go back and look - is it confirmed that the king has multiple concubines or something? I guess if the crown prince is the same age as Tiara, there would have to be because otherwise he's definitely not the queen's if she's pregnant with Tiara at that time (unless he's 1-2 years older than the other main characters, then he could be the queen and king's without issue).
Basically I took it to mean "the actual queen is Tiara's mother, making her royalty - but she was born a bastard child because the queen had an affair. To hide this fact, Tiara was passed off as being the handmaid's child instead, until it was determined it was safer to kill Tiara to mitigate the risk of the queen's infidelity being found out - plus to cut off any line of inquiry about Tiara's potential place in royal succession".
At that point, the handmaid ran off with Tiara to save her life, successfully hid her in an orphanage, and was subsequently caught and executed with Tiara left to grow up a commoner, none the wiser.