My parents remarried, and it turns out my two stepsisters were actually together! I thought they just had a good relationship, but since they're sisters, I didn't think much of it. Turns out their relationship went beyond siblings! They'd get cozy when my parents and I weren't looking, and when no one was around, they'd have intense lesbian sex! It was a taboo-filled, intense lesbian relationship that couldn't be found out by the family, but I happened to catch them. Then they asked me to keep it a secret from our parents and used sex to seal my lips, leading to a threesome! In the end, I too got sucked into a forbidden relationship with my relatives!
