Selene had the same reaction as Azrael when she saw Sienna. She was starstruck by her angelic beauty. Indeed, angels were all beautiful creatures.
She already knew how handsome an angel could be because of Eros. But she didn't expect that a female angel would be this breathtakingly beautiful.
If Aphrodite is the goddess of beauty in Greek mythology and Venus in Roman, then Sienna is the goddess of beauty in the Angel's Agape Realm.
She was wearing all white. It was a simple white dress but Sienna looked very stunning and elegant with that plain white dress. She assumed that Sienna's hair would be smooth and soft by just looking at them.
Sienna already hid her wings even before she entered that room so Selene didn't see her beautiful silvery-white wings. She would be more fascinated once she saw it.