Debcz, my heart goes out to you! How can a D be so callous towards a M? And how can parents be so insensitive to the feelings of their own children? How can they teat kids away from GPs that they love and that have been a big part of their lives? How can they tell kids that someone they love is a "nasty person?" I don't get it.
I agree that there is more going on here than the rehearsal issue. And for a D to laugh at her M's pain? Well.. I imagine this is about more than wanting to sell property, also. I suspect there is an ongoing issue between you and D, unforutnately, even if you don't realize it. Maybe she didn't feel she could react to it too much while they lived on your property, but now that they moved, she may feel she can. Or perhaps the rehearsal incident was the proverbial "last straw" to whatever overall issue is. Did SIL give you any clue to what that larger issue might be when he told you why he was taking Freddie away from you?
He did let you know what you can do to regain access to your GC - apologize. The question is, for what? For going to the rehearsal w/o letting them know/giving them a chance to ask you not to? That's easy enough to do, IMO. Or for "sneaking in?" That would be harder, I imagine, since that's not what happened. I think it's worth apologizing for going w/o discussing it w/ them first, as it involves their children. But that's as far as I would go.
You may be afraid the same thing could happen again if you do something D doesn't like. But IMO, maintaining you relationship w/ your GC makes it worth a try. If it becomes a repeat pattern, you can rethink.... Just my opinion...