Netorare Boukan Nikki ~Hentai Ishi no Seikenkyuu no Ejiki to Nari Risei no Kamen o Hagarete Yogari Nakasareru Ore no Imouto to Osananajimi to Sensei~

The protagonist was a regular student (male). His strict role model female teacher, tender and shy female friend and super cute imouto were all part of a fun academic life.

But one day something strange happened to his teacher. It seemed to be an airborne infection. Then it spread to his friend and sister. They all became.. strange. It wasn't clear, but the girls were... different.

To learn what's come over them, he followed them...

[From DLsite English]