Both my wife and I as wife's sister and her husband like hanging out naked from time to time.
First started by going to public nude sauna centers and nude beaches.
However, SiL did not want us to hang out naked together. She thought it would be awkward when her BiL got to see her naked and vice versa.
However, SiL husband is a real nude lover. He walks around the house naked almost all the time.
Even got caught by MiL few times.
So we all had seen him naked a few times, but none of the rest had seen each other naked.
Untill some years later. The 4 of us were on a weekend away during winter without kids.
We rented a cabin that also included a sauna.
After a day of hiking, the guys went into the sauna and afterwards the girls while we prepared dinner.
The fireplace was burning quit wel, thus it became quit warm in the cabin.
Not ideal when you just came out of a sauna and want to cool down (even after taking a shower).
So instead of wearing a bathrobe, us guys decided to be naked.
After the ladies walked in, they were a bit surprised to see the both of us naked.
It was the 1st time my SiL got to see me naked ... and for my BiL ... well we all had seen him naked more often.
Turned out that size wize, both my BiL and I were in the same department. About 7" and clean shaven.
Only difference is that he's cut and I'm not.
After dinner we we're playing a board game. The guys still naked, the girls wearing a bathrobe with a nighty underneath. All of a sudden, my wife said "it's not fair that you guys get to walk around naked while we get to stay dressed. It's too hot in this room to do so". Without even looking round or asking, she stood up and removed her bathrobe. But she didn't stop there. To my amazement, she also removed her nighty, so she was standing stark naked in front of us. That was the 1st time my BiL got to see my wife naked.
At that moment, 3 out of 4 were naked, but my SiL refused to go naked too.
Afterwards my wife did confess she did it because she was feeling a bit frisky.
Bit of alcohol and the sight of 2 naked guys did the trick.
Somehow the action of my wife had triggered something, because 2 weeks later, my SIL agreed that we all would go to a sauna center together. So I finally got to see her naked
... and my BiL and I could compare the looks of the 2 sisters.
But all has happened in a non-erotic, pure platonic way. And I don't think anything more will come from it.
I'm personally more in favour of smaller breast sizes, so although my SiL has a nice pair of DD's (vs my wife's C), she's not my type of girl
But on the other hand, I'm glad that it all evolved like it did, cause also our kids like hanging out naked on the beach and going to nude swim events. Hopefully they keep enjoying it, cause they are all their early teens.