Just to make it clear, she was my EX. We were more than just through at the time. Yeah, sleeping with her sister was below the belt.
My ex and I had met with her introducing me to her sister (as a boyfriend match-up). Ex then decided to keep me for herself, her and sis had a huge fight over me at the time. I had already hooked up with ex 2 days after she had introduced me, trying to hook up with her little sister then would have felt more than awkward.
I could have banged her sister for 2 years while we were together, but didn't. After what she put me through at the end, she had it coming.
Consider that ex is not on speaking terms with most of the members of her own family. While I am practically treated as family by them - that is after sleeping with her sister.
b0und (judge all you want)