My plain and inconspicuous sister-in-law has hidden big breasts. What's more, she can't say no when asked to do something. I actually have feelings for her. However, my playful friend makes a joke that is a bit sexually harassing, "Your boobs are big, please touch them," and she takes it seriously and lets him touch her breasts. Finally, when I get home, I see my sister-in-law having sex with my friend. To be honest, I'm shocked... and I'm depressed. But since she can't say no, I decide to ask her for sex!