I've been fuck buddies with my sister's boyfriend for the past few months and now he wants to stop.

This is a long story, so let me (18F) start this off by saying I never meant for any of this to happen, or at least spin this much out of control. My sister's (F25) boyfriend (M29) and I have been fuck buddies since I turned 18 last March. I wasn't attracted to him at first, but the way he's such a gentleman with my family and witnessing how he dealt with my sister's rough personality made me take notice. He tells my sister, who's a bit of a bully, to be kinder to me and to spend more time with my parents, and always makes sure she's home safe by bringing her home every time they go out. My parents liked him so much, they started letting him use the guest room whenever it's too late at night after bringing my sister home. And that's where the trouble started.

My boyfriend (M32) and I just got into a huge fight and I was emotionally vulnerable that night. I saw that he was asleep in the guest room with just his boxers on, and at that moment he was everything I wanted to have but couldn't. He was kinder, more mature, sweeter, and much more caring than my boyfriend despite him being younger.

I started kissing him while he was sleeping, sucked on his nipples, and slowly took off his boxers. His dick was HUGE. It was hard already from me nibbling on him, so when I held it, I was shocked. My boyfriend was around 3.5" at full hardness, but I could double that and this dick I'm holding would still be bigger. So I got snuck into his blanket and started sucking him off. My eyes were watering from the size of his dick and I loved it. He woke up, but since he thought I was my sister, he just kept moaning "yes honey" and I kept going until he filled my mouth up with his sperm. I swallowed everything and then I came out of the blanket, and he was understandably shocked to see it was me.

I remember one of my sister's very few complaints about him was his insatiable sex drive - like he needed sex almost every day, and sometimes my sister would come home really tired because they spent the entire day having sex. Keeping this in mind, I begged him to keep our little incident a secret, and then I asked him if we could be fuck buddies. No feelings and shit, just let our overflowing lust out onto each other. I said he could do anything to me, including the things he wants to do to my sister but can't. When I said this, he finally agreed.

I spent the next few months letting him have his way with me. We did everything he wanted - BDSM, CNC, creampies every time we fuck (he has a huge breeding kink), anal, anal creampies, roleplay with costumes, sex in public, sex in his office, in my classroom, condo balcony, golf kart, what have you. I said yes whenever he wanted to video us (last time I checked, we had 126 videos already). I even got 4 of my classmates to check in with us once because he wanted to experience a reverse gangbang. Basically I gave him everything he wanted, and in exchange, I got to have 8 inches of thick, throbbing manmeat whenever I was itching to be railed. I stopped having sex with my boyfriend also, since I couldn't feel anything anymore even when he was thrusting so hard.

He stays for some cuddles when I specifically ask for it, but never talks about anything deep or important - it's always the same topics like the food we ate, the weather, NBA, politics, but nothing close to him. Even when I try to get him to open up, I get nothing.

And this is where shit hits the fan: Last week, he was gone for the full week due to a spiritual retreat somewhere in the province. And when he came back, he asked to see me. Me, missing him after not getting railed for a week, got very excited and put on my best lingerie and perfume, only for him to tell me he wants to stop our arrangement permanently. He said the guilt has been eating away at him for a while, and he can't continue anymore because he is planning to propose, and wants to do right by my sister. He said we can tell her together, or he'll tell her on his own, but he can't keep lying to her anymore so he'll admit everything and if she dumps him, he knows he deserves it. He says he'll give me a week to decide.

I don't know what to do. This has grown into something I didn't expect. I'm also feeling guilty now. I asked if we could just end things without telling my sister but he doesn't want that. If they get through it and eventually marry, that means I'll never see them again. If they break up, I'd have destroyed their relationship but he'll be free to be with me. I don't know if he'll want to but the option's there :( I wish we could just go back to the night I saw him sleeping and stop myself from crawling into bed with him.

My situation's impossible, but redditors, any advice?