Finally have a bit of spare time.
Hopefully my post doesn't vanish into thin air this time!
So, we arranged with sil and her hubby to come round one evening and stay the night.
We told them in advance that nothing would happen more than they wanted. Either of them can halt the "proceedings" at any time with no hard feelings and no pressure to even follow up in the future.
When they came around we had a quick snack (just tea/coffee and some cakes and then went to sit in the living room.
We had agreed in advance that on this occasion there would be no "sharing" only watching.
So, we sat there on one couch and they sat opposite us on the other.
Wife and I started to make out and I could see that they were hesitant to start with. Soon, they also got into the spirit and also started to make out. At this point both couple were fully dressed.
Slowly clothes started to be shed on both couches.
We could here the moans coming from the others so they were clearly enjoying it. And they were definitely watching us too.
This continued until both couples were down to our underwear. It was obvious that SIL's husband was very much enjoying it.
SIL was wearing a very plain set of underwear. Something that was definitely created to cover rather than arouse. My wife, on the other hand, was wearing some nice frilly underwear which was the exact opposite. Not much was hidden.
At this point SIL decided she wanted to stop. She said she was okay with what had happened so far but didn't want to go further - at least at this point. As we had promised, we immediately stopped (difficult as it was).
I could see that her husband was a bit disappointed so I took him aside and told him that pushing it would be far worse. Let her do it at her own speed our they would end up not doing at all.
She may not have been willing to carry on in public but from what we could hear when they were in bed, she was definitely NOT holding back when they were alone.
Wife and I took this as a good sign, since SIL was obviously turned on by the whole thing.