This story didn't start today; this sexual tension between my sister-in-law and me has been going on for a while! My girlfriend can't find out about all the times her sister flirts with me because she'd go crazy! Tonight we were playing board games before going out, and my girlfriend went to take a shower, leaving me alone with her sister! My sister-in-law didn't hesitate for a second before she started provoking me to fuck her. I resisted as much as I could, but it happened! She sucked my dick, and I fucked her doggy-style while my girlfriend was showering in the next room. She almost caught us while we were fucking!