FSDSS-392 My lustful lover's younger sister wanted me to take a bath with her and that was the end

Natsuki Rin has a boyfriend but she is not very satisfied with him because he is gentle and stupid, but that doesn't make her leave him until one day she meets her lover's friend, he more handsome, more agile than her boyfriend and more delicious with a skillful tongue, making this girlfriend fall in love and give her ex-boyfriend a pair of horns