Alone with my wife's sister

I'd like to share a story that happened to me six months ago. My wife, Katya, and I have been married for six years. She's a beauty: slender, with long blonde hair and a sweet smile that makes everyone melt. But her younger sister, Vika, is a whole other level. Vika is twenty-seven, three years younger than Katya, and looks like an Instagram model: long dark hair, big brown eyes, C-cup breasts that always seem to burst from their cleavage, and a firm butt. She and Katya have similar faces, but Vika is brighter and bolder. I always tried not to stare at her too long when she came to visit, but sometimes my fantasies would creep in—especially after a few drinks. Vika lived in another city and rarely visited, but this time she flew in for a whole week—she was on vacation, and Katya had just left for a five-day business trip on the other side of the country. My wife and I saw her off at the airport. Katya kissed me and jokingly said, "Keep an eye on your sister, just don't stare!" and laughed. I smiled back, but something inside me sank. Vika stayed at our house, in the guest room. That first evening, everything was decent: we ordered sushi, opened a bottle of wine, chatted about life, her work, and how she'd recently broken up with her boyfriend. She sat across from me in short shorts and a tank top without a bra—her nipples were slightly visible through the thin fabric, and I caught my gaze constantly sliding down. On the second day, we decided to cook dinner together. I was chopping vegetables, she stood next to me, bending over to pick up spices, and her breasts accidentally brushed against my hand. The atmosphere was getting tense. "You know," she suddenly said, pouring us a second round, "Katya always bragged about how great her husband was. And I watched and thought... she wasn't exaggerating." I laughed, but I felt my cock stir in my pants. We drank some more, turned on some music, and she pulled me into a dance right there in the kitchen. Her body pressed closer to me than necessary, her hands slid down my back, and then lower—to my butt. "Vika..." I began, but she put a finger to my lips. "Shh. No one will know. I've wanted you for a long time." We found ourselves in the bedroom—in Katya's and my bed. She pulled my T-shirt off, ran her nails down my chest, and sank to her knees. Her lips wrapped around my cock—hot, wet, she sucked deeply, looking into my eyes. I couldn't resist, lifted her up, tore off her shorts and panties—her pussy was already wet, smoothly shaved, with small pink lips. I laid her on the bed, spread her legs, and began licking her. She moaned loudly, holding my hair: "Yes, like that, with her tongue too... oh god, keep going!" She came quickly, squeezing her thighs around my head, and immediately pulled me up: "Enter me. Now." I thrust in hard—she was tight and hot, wrapping her legs around me and whispering dirty words in my ear: "Fuck me harder." We changed positions: first missionary, then she straddled me, riding me so hard her breasts bounced in front of my face—I sucked and bit her nipples, and she scratched my chest. Then we were doggy style in front of the mirror—she looked at our reflection and moaned. I spanked her ass, and she squealed with pleasure. When I said I was going to cum, she turned around, took my cock in her mouth, and swallowed all the cum. Afterwards, we lay hugging, sweaty and content. "This stays between us," she whispered. "But I want more... before Katya comes back." And we continued all week. During the day, she walked around the house almost naked—only in my T-shirts, which barely covered her butt. We fucked in the kitchen, in the shower, even on the living room couch. One time, she came to my office during lunch—in a short dress without panties—and gave me a blow job under the table while I pretended to work. Another evening, we had a "massage session" that ended with me taking her in the ass—she gasped at first, but then begged for more, coming so hard her legs were shaking. When Katya returned, Vika acted as if nothing had happened—hugging her sister and telling her about her "boring week." And I looked at them both and thought, "Same blood, but completely different feelings." Vika left, but now we secretly text each other. Sometimes she sends photos of me in lingerie with the caption, "Do you miss your sister-in-law?" Katya suspects nothing, and that makes everything even hotter. I know it's wrong, but I can't stop. My sister-in-law is forbidden fruit that turns out to be the sweetest thing in the world.