Lovely wife you have there.
It is totally normal to be apprehensive when she has someone without you being there - the cuckold angst of knowing what she is doing, not being able to stop it, and at the same time not actually being able to see what she is doing at that moment. The mind goes into overdrive.
The transition from hoping it will happen, to it actually happening is nerve wracking, you get such intense feelings inside. It sounds like all of you want this to happen, and I am sure you won't regret it once it has happened.
My wife always excluded me from the house when she was with our best friend, it was so horny for me, knowing they were fucking together in our bed. The first time was so special, so exciting, we made love directly he had left, and it was the most intense lovemaking session we have had.
Let them have space to explore alone, she will love you so much more for it.