A minute detail I noticed is that the sword that each twin yields is different. In a way, Lumine's is in shape of a moon and Aether's in shape of a star? :
This sword fits her so well- : r/Genshin_Impact
Genshin Impact Player Female Traveler Lumine Sword Cosplay Weapon Prop
Épées originales du voyageur Éther et lumine Genshin Impact Prête à être imprimée en deux parties - Etsy France
lumine (genshin impact) drawn by hatsuga_(dmaigmai) | Danbooru
Lumine vs Aether - New Swords & Voice Showcase We Will Be Reunited Story Genshin Impact - YouTube
Genshin Impact Lumine Best Build, Skills, Talents, Age & Back Story