Great story - recall reading that last year, and is similar to this current developing situation. Though is slightly different in that it occured while Steve and his wife were together visiting the sister, vs when the the OP is visiting his GF's sister when his GF is not there...
The OP might want to think about where this is all heading with his GF's sister, and his GF for that matter. The sister obviously wants some male attention, and has found a willing, and apparently "safe" audience [what breathing male would not be willing??
] Does she have ideas of taking things further, is she looking for a new partner to help with her kids and is she thinking her sister's BF might be a good catch, or is she just being flirty and casual about her nudity for some light fun in the here and now? Does the GF know about her sister's now pretty blatant exhibition and would the GF be OK knowing about the recent developments, or would she go ballistic? The OP said the sister had acted differently when his GF was there with him.
It could remain a fun little flashing game between the sister and the OP or it could evolve into something much more, but know I would have difficulty keeping it eyes only. Years ago had a GF who also had a younger sister, and though never saw her nude [though many DB and upskirt views] did have more than a few mutual close physical explorations under the guise of tickle "fights," sometimes right in front of my GF - she ignored our "horseplay" as it evolved from innocent to not so... The sister was a toned, DD cup cheerleader and my GF was a B, so the allure to touch, both high and low, was just too much. Nothing was ever said, at least by me, and it was never discussed. It remained our little game for years.
