https://www.dlsite.com/girls-pro/work/=/product_id/BJ053937.html
https://book.dmm.co.jp/product/825977/b403assog03319/

Description:

Mika and Yuka are twin sisters. One day, Mika went to pick up Yuka later than usual as she'd gotten delayed due to a committee meeting, but suddenly she heard Yuka's panting voice. Even a late bloomer like her realized immediately the tone of embarrassment in her sister's voice.

"Is that what you call a French kiss?" I wondered. With their faces tilted, it was as if they were tasting each other. My teacher, whom I also yearned for, held Yuka's breasts lovingly as he began to massage them, eliciting a seductive moan from her. Witnessing that sight, my own nipples harden as though I am the one being massaged by him.

Brushing off her embarrassed appeals as she lies on the teacher's desk, my teacher spreads open Yuka's legs and licks her center noisily. "Is that cunnilingus?" I wondered.

"You're already overflowing with nectar!" my teacher proclaims to Yuka.
"S-stop teasing me!" she responds.
Enticed by her moans, he finally enters her!