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The older brother is aroused by his tsundere younger sister's defenseless panty shot. They both get excited when she shows her sex with her boyfriend to the older brother. When he sees her having sex with her boyfriend choking her, he finds out that she is a masochist. In the bath, she begs him to wash her. The brother and sister lock themselves in the younger sister's room so that the family doesn't find out, and start having sex, remembering the times they used to touch each other.