I came across this thread only recently.
Since Grits mentioned in-law swapping I thought I should tell our story.
Wife and I have been swapping for many years with close friends. They were close before it started and even more so now.
So, about 3 years or so ago my wife was having a conversation with her sister. We are both in out mid 50s and sister is 3 years younger. I wasn't there so the quote is hearsay.
Wife's sister was complaining that sex was getting very boring and, though not a "chore", they were not enjoying it as much. She asked my wife if the same was true for us and wife replied that ours is awesome.
Sis-in-law asked for our "secret". At first wife didn't want to say but eventually she told her.
From what my wife said her sister was very surprised (to say the least).
When she asked how she could let her husband (me) fuck someone else and her husband (me) allow her to fuck another woman, she explained that it is not a matter of "letting". Neither of us owns the other one. It's more a matter of wanting to make your spouse happy.
Her sister said that there was no way her husband would be open to anything like that to which my wife replied "You'll be surprised what guys want to do".
Sis agreed to allow me to somehow bring it up with her hubby and see what happened.
I got into a conversation with him about life, the universe and everything and slowly managed to bring the conversation around to their sex life. In the end I had the same sort of conversation with him as my wife had had with his wife.
He agreed that they would be open to hearing more about how it works with no promises.
So, we sat down with them and explained how it has to be 100% agreed from both of them. If one of them had any doubt then it would result in disaster. The important thing to take into account was are you okay with your wife doing something that will give her great pleasure.
If the answer is yes then you are on the road.
Their next question was about embarrassment of being with a strange couple. Especially is the other couple is experienced.
I looked at my wife and could see she was thinking the same thing I was.
I suggested that instead of a strangers they could start with someone they know well.
They were a bit hesitant and we gave them time to discuss it alone.
They came back with an okay but they wanted to do it slowly.
Over the next few weeks we worked up slowly - stage by stage. We didn't want to rush them and they wanted to pace it.
In the end it was well worth it.
Having sex with your wife's sister is something special. Hard to explain if you haven't done it yourself.
Since then we have also introduced them to the couple we regularly share with and many times have a mini-party together.