@McNANA Rain probably did report it but the crown prince was too sure to believe it wasn't anyone else but Mielle, because remember in chapter 27 Asher whispers to Aira ' you're quite different than what you appear to be' implying she's acting naive in front of the others. Or probably Rain on his part didn't report things proper even though he realised Aira is the sharper one & Mielle could only come up with garbage, he didn't suggest looking into the matter
To thoes saying Asher couldn't tell Mielle and Aria apart, he first met Aria alone in the casino shop and mistook her for Mielle. I doubt he had ever met Mielle before (or even both sisters together) to tell them apart