My sisters BF touched my ass and now i don’t know what to do

My(18F) sister Sandy 24F (fake name) has a boyfriend John 27M (again fake name to conceal identities) and they have been together for 3 years now.

I’ve always been friendly with John, he is good to my sister and treats me well, pays for when we all go out, cooks for all of us sometimes etc.

Recently the three of us went to a vacation on an island, i won’t bore you with all the details but it was an experience of an lifetime, great weather, amazing food except for one thing which has been bothering me a lot.

On the third night we went out drinking and partying, Sandy had waaaay to much to drink, she started clearly emmarasing John in the club, he tried to ignore it for a bit and we actually danced for a while trying to have fun but Sandy was too much so we all went home and put her to bed.

We are all in one big room and after we put Sandy to be John and I went on balcony to smoke a joint, we had a great time and i don’t know if it was the alchocol or what but i noticed he started being flirty and i noticed i was being flirty back.

After an hour or so we went to our beds, i noticed John stripped down to his boxers and i purposely changed out of my clothes in the room. So if he was looking he could see me in my panties and could see my bare back since i was changing into a sleeping t-shirt.

That was it for the night, a bit strange i though, but alright, we were drunk and high, little bit of flirting doesn’t hurt anyone.

But.

The next day, we planned to go to the beach but Sandy was feeling as hungover as anyone would, we wanted to all stay in since we were also kinda hungover and wanted to take care of her, but she insisted we go to the beach and not waste a day.

Again to spare you the boring details beach was super nice, so much wildlife you can see, you feel like you are in a jungle. Since the sun was high in the sky we needed sunscreen, i put it on myself where i could but then it came my back and hard to reach places.

Here comes the crazy part, i ask John to put the suncreen on me, he more than happily obliges i go on my stomach and he at first starts spreading it on my shoulder, back etc.

Then he asks if he should do my legs. My head starts kind of spinning, i already did my legs, he probably saw i did them as well. But he is asking if he should do them again. I say yes. He starts from my soles, working on them, rubbing it real good, it felt more like a massage then just putting on the sunscreen.

Soon he starts going higher and higher and he ends up on my thighs, now its hot outside, but not as hot as i was feeling in that moment. He starts going deeper and deeper, i am already breathing hard.

At one point he goes so high up he touches my panties and i make a little squeek, John asks me if I am okay, i respond with a little yes with a dry throat. He countinues, what feels like almost rubbing my pussy, he goes down pretends its a normal massage, then reaches up and catches a glimpse of my pussy.

At this point i am dripping, there is no way he doesn’t feel this. I start thinking about Sandy, it doesn’t feel fair to her. I gather courage to tell him to finish it up and this guy just says „sure“, i think i am in the clear but then he puts more lotion on his hands and starts putting it on my ass.

I am ready to explode any time in that moment. I’ve never felt more turned on, if he wasn’t my sisters boyfriends i would take him that same second.

After a few minutes of rubbing and putting on the motion he slaps my ass and says done, with more confidence than his face would allow.

I turn around and thank him, his boner as visible as clear sky was on that day. John says he is going for a swim and I countinue laying there, thinking about his hands touching me and his huge dick against his swimming trunks, dying inside, knowing what i want, but knowing its so wrong.

The rest of the vacation John acted like nothing happened, to be fair I did too, but I could feel the tension, i could see the looks we would give eachother.

Now we are back home and i just don’t know how to act, what to do, i feel frustrated in so many ways. I rubbed myself thinking of his touch more than dozen times, i just don’t know what to do.