If I understand this correctly, Stheno's upset that Medusa couldn't tell the difference between them, unlike Asterios.
Huh!? Eldest Sister?Hmm...what is up with that big lug anyway?AhSecond Elder Sister! Did you call for me?Meduseless.
It's a pun on Medusa's name here. She calls her "駄目デゥーサ" "Damedusa" which is basically "Useless" tacked onto the front of her name.
