Did I miss something? Did your best friend do something more than hang out with your SIL?
If she didn't she is your best friend of twenty years Steph. Twenty years of life - memories - great times - hard times. I am sure you have been through it all. I honestly think you will regret these choices years from now. Slow down.
And many of us have noted that they both have time on their hands. I think you even mentioned that as a reason that they are going out a lot. Because they both have the time.
You have way too many emotions going through you surrounding all this situation. And I believe it is clouding your judgment. I would also stop involving others, way too messy. And merely causing drama. People talk. I know you say you are looking for advice. But to me it feels like you are looking for more back-up for your own thoughts, decisions, emotions.
Stop. Take a breath. This is your friend. Just continue being a friend to her. I would never cancel a trip with my friend over this either. How do you think she will feel about that decision? Have you really given her side a lot of thought about that decision? That would be a struggle for her afterwards regarding your friendship, don't you think? This is what she would be dealing with - My best friend said she doesn't want to go away with me anymore because I went out with her SIL. Doesn't that seem bizarre?
I honestly believe that you simply have a boatload of emotions going on around this Steph. Some of which probably have nada to do with your best friend. About your SIL, maybe family dynamics your own stress....
And you are looking to a quick way of dealing with all these emotions by placing it all on your friendship. People do this all the time. Too many emotions - find a way to squash them instead of simply dealing with them.
If you simply can't deal with it all, all I would say - and in a casual manner - is *I am emotionally struggling with this Lisa, just wanted to be upfront with you because obviously you are __________ (whatever she personally means to you in life etc)*. That's it. And that's only if you think you are going to become distant or passive aggressive with her because you have so many emotions surrounding the situation.