[Moririn-Monson] Kanojo no Gal na Onee-san ni Sasowareta node Shimashita. Ch. 4 | My Girlfriend's Gal-like Onee-san Seduced Me and We had Sex Ch. 4 [English] [desudesu] [Digital]
I'm really surprised the girlfriend hasn't suspected a thing up to now, considering she keeps finding her boyfriend together with her sister everywhere.
After 4 chapters this become, the older sister is the real girlfriend, the younger sister is the mother of the girlfriend, not much point of the younger sister appearance.