"Oh shit. We didn't realize you two were here. We'll go."
It's 11:30 at night. And that is my wife's brother and his new live-in girlfriend. And they had just walked in on us sitting in a hot tub at a house party. We live in a small city, and 3 years her junior, they shared mutual friends. She is a good big sister. Really, really good.
"Don't be silly brother. Come in. There's plenty of space."
It was a hot summer evening and an all day BBQ and party had started to wind down. Anybody that had stayed, were inside drinking and being loud. Her and I snuck out to the patio, stripped down to our underwear to have a soak. She was radiant. Long, silky black hair that floated on top of the bubbling water. Her little see thru black bra, there in principle. But effectively pointless for modesty's sake. And little black thong panties. On its own, it was practically a bikini, minus the fact her thick nipples showed through. As did her chest tattoos.
Sitting in that deep tub, she was somewhat covered. I also didn't mind who saw her. It was fun to show her off. I was proud of her beauty, if not objectified it at times. And clearly she didn't mind either. It was fun. What I hadn't accounted for though, was her brother showing up. Or at least, him and his girl also having a soak in mind. They both came prepared in their bathing suits. He has a nice physique and his girl Amy is quite thin, covered in tattoos (as well), and has these tiny little titties that hid behind her bikini top. She is cute. Much less voluptuous than my normal taste in women, but I couldn't help but feel my cock twitch when I saw them both step into the hot tub with us.
The four of sat there, chatting away and drinking beer. It was innocent enough, outside of the setting we were in. The girls started talking about their tattoos, which soon became show and tell. They both had new pieces, and perhaps the alcohol and the safe setting spurred on questionable decisions. Or maybe they knew exactly what they were doing.
"Is that new?" Amy asked my wife pointing to her shoulder and the front of her chest.
My wife smiled, covering her tits with one arm, and sat more upright to show off the new piece that adorned her body. She pulled her see thru bra up over her nipples, still being careful to cover herself the best she could, to show how it extended underneath her boobs.
Her brother and I both sat there, not saying a whole lot. I imagine he felt as I did, albeit much more conflicted. Amy giggled and undid her bikini top, spinning around to show a large back piece that also came around her rib cage and under her own little A cup tits. Once she gave us all a good long look, she dipped back down below the water line, but now, fully topless.
My hand under the water gave my wife thigh an encouraging squeeze. As if I had straight up asked, quickly her own bra came off and just as quickly, her large boobs disappear under the water. The four of us, now feeling loose and sitting much closer to each other, started laughing and eased back into random conversation. My cock was swelling uncomfortably in my underwear. It was overwhelming sitting there with two topless women and a sibling. Before long, the beer was done and more was needed.

"I have to go pee." my wife said, and without missing a beat, her big curvy body stood up and she got out of the hot tub. Water cascading off her chest and her round ass framed so elegantly in her thong. I was dying. 

"I'll bring some beer back too." she politely offered.
Lost in thought, I slouch in the water, trying to hide my erection. Not that it was noticed. Amy and my wife's brother were giggling to themselves, and started some gentle kissing, before engaging back with me in idle talk.
A few minutes later, my wife comes walking back on the patio. It was one thing her standing up naked and her big ass shown to the three of us as she got out. It was entirely another when she arrived holding multiples beers and no ability to cover up. My cock was unapologetically hard now. What was even happening?
My wife climbed back in the tub and handed us all a beer, before sitting on the edge, not even in the water. She cracked hers opens and announced how hot she was feeling. She just sat there. Her chest hanging, her little tummy rolls creased, and her perfect ass spilling over her thong.
