My sister-in-law doesn't wear underwear

I and Rio have been dating for three months. This is the first time I've been to her house and also the first time I've met her sister, Anna. She has a beautiful face and an attractive figure. While we were studying, she suddenly lifted her leg to reveal the space between her thighs. What? No underwear?! That's right! I could clearly see her beautiful genital slit! She intentionally opened it wider and posed in ways to provoke me.

And when my girlfriend was absent, she approached me and seduced me skillfully. I completely lost my mind, and my penis couldn't stop penetrating deeply into that enticing slit. I love my girlfriend, but her sister gives me pleasure I've never known before. And since then, both of us have been secretly having sex whenever my girlfriend is not around...