A husband and wife both in their 50s who are into some light discipline play. blindfolding, handcuffs, spanking by hand and occasionally with a belt. nothing heavy or dangerous, no actual humiliation just mild domination and light punishment to enhance sex, the sex is more important that the spankings but the submission get the wife so horny the sex is so much more intense when they indulge in some discipline. then One weekend the younger sister of the wife and her husband come to stay the sister late on the first night goes to the bathroom and overhears what is happening in the husband and wives room, the door is slightly ajar and she sees her sister over her brother in laws knee in the mirror being spanked the fondled, spanked then fingered and cant help but watch and touch herself. Next day she confesses to her sister what she seen and how it affected to her, without the men knowing about that so far