I fucked my girlfriends younger sister and I think she knows

So I (20M) and my gf (20F) have been dating for around 2 years now. The connection has always been there, but more recently the sex has gotten pretty stale. I have a very high sex drive, and had my fair share of fun before locking things down with her. My gf on the other hand, was never near as sexual as I am. I’m her only partner ever and for the first year or so the sex was amazing, hot and intimate sex once or twice almost every day. I couldn’t get enough of her body and tight pussy. But now, this far into the relationship, I’m getting a bit frustrated as the drive isn’t what it used to be. I’d be lucky if we got to fuck once a week. Since I’m in the prime of my life, I have taken to things like Reddit in order to ease my cravings, but it hasn’t been enough. Her younger sister (19F) is very attractive and begun going out and partying much more as she has begun college. Everything happened when I went home to visit last spring break. My sister wasn’t home yet (the days didn’t overlap perfectly), but I decided I would wait for her to come back from the airport at her house, where her sister happened to be. We were alone since her parents went to pick her up, and at first it was awkward small talk like “how’s school been?” And all that stuff. But then she sat next to me on the couch. She started laughing harder at any little joke I would crack and slap her hand on my shoulder and squeeze as she laughed. I knew this was getting out of hand, but I had been pent up for so long, and her slender blonde sister was looking so amazing. I leaned in for the kiss and she met me back with one. My heart dropped from the nerves and guilty but my cock throbbed. She was quick to take of my shirt, followed by hers, where I started playing with her perky set of tits and grinding on her pussy. After a minute of this, I took of my pants and she got on her hands and knees and sucked me off while I sat on the couch. I honestly thought I would finish right then, her head was amazing and experienced. her tongue slid up and down my shaft while she made direct eye contact with me.The handwork and suction was so much better than my gf had ever given me, which made me feel guilty, but it felt too good to stop. I made her get up on the couch and 69 with me. Her pussy tasted like heaven, it was tight, pink, and absolutely drenched. I knew in that moment I need to fuck her before I came. After about a minute, I got out from under her and went in back of her in doggy style and teased the head of my dick in and out of her pussy until she begged me to stick it. With a large thrust, I went balls deep inside her and she let out the hottest moan. Her pussy was so tight and warm, every stroke her pussy would grip the shaft of my cock. Finally, I was getting close to finishing. I was stroking faster and harder until I could barely handle it anymore. I told her I was about to cum and she told me to pull out, which I finally did by popping my cock out of her ruined pussy and letting off several cum shots all over her back. She then went to go shower and I put my pants on, guilty, nervous, but so satisfied and aroused. About 10 minutes late, my gf and her parents walked in the door. We hugged and kissed, but she seemed to know something was up. I reeked of sex, and her sister was in the shower the minute she came back. As of late my gf is acting wierd, but hasn’t said anything. I still feel guilty, but me and her sister still sext each other and plan on fucking more when I come back. Honestly the best and worst decision of my life.