I don't remember what triggered it, but my sister and I have been in a romantic relationship since our student days.
Before either of us had boyfriends or girlfriends, we secretly did pleasurable things without telling our mother. However, my sister gradually started to resist.
But because my sister was sensitive about her nipples, if she wasn't in the mood, I could make her compliant by tormenting her nipples. Originally, my sister was sweet to me as her younger brother, so I kept having sex with her until I was satisfied…
