I really feel for you. Being left out with a very closed loved one hurts terribly. It hurts even more when it comes on top of other recent traumas and when others are not left out and when it's unexpected.
That said, first, I'd take a deep breath and try to let this go for at least a couple of months. As I'm sure you know, since you brought it up here and not with your daughter, engaging in any kind of upset with your daughter right now will only make everything worse.
I think it's quite possible there's been a misunderstanding. For ex., the couple wanted this to be with their new little family only, but weren't assertive enough to say it. But the other grandmother could have noticed what was going on next door and included herself, with your daughter and her husband simply feeling unable to ask her to leave. That's one possible explanation and I'm sure there are others.
If it was not some kind of misunderstanding, then it's possible the relationship between you and your daughter isn't actually as good from her perspective as it is from your perspective.
Imo it's not all that uncommon for one party in a relationship to be shocked to discover that the other party doesn't feel the same way they do about the relationship. Or at least to be shocked to discover the extent of the other person's negative feelings, since we all know no relationship is perfect and we must all tolerate some imperfection. A grown child cuts a parent off, a spouse walks out, a boss fires you, all seeming to the person on the receiving end like it came out of nowhere.
But imo perhaps more likely, the "bomb" finally drops when the "bombing" party can't take any more (regardless of each side's actual right-doing and wrong-doing in that relationship. Maybe some people are just more resilient than others, among other possibilities there).
Then we want to lay blame on one side or the other, for the relationship problems and also for how the dissatisfaction was handled. "I sacrificed so much for you when you were growing up and you're ungrateful." "You never listened to me." "If you were so unhappy, why didn't you say so?" "Well, you did x, y, z." "Well, you didn't do x, y, z." "It's your spouse's fault, the in-laws' influence, etc."
But maybe it's more accurate and fair to say the relationship was dysfunctional, to some extent at least, but everybody kept going with it because that's just what we often do, and then it finally reached a level for one party that it broke apart or, more fortunately for those who want to repair it, started showed cracks.
So often on these forums, we see estranged or semi-estranged parents who can't get over trying to "prove" why they are right and their grown child is wrong. And imo they're also the ones who can never get their relationships back on track.
Of course, this can't be anything but my general thoughts since I don't know you or the situation beyond what you've said here so far. But if it comes out that your daughter actually is unhappy with your relationship, my advice is to take a deep breath, resist the urge to defend yourself, thank her for sharing her thoughts with you, and then stop talking for the moment and get a therapist involved ASAP.
If it got this off-track without you being aware of the extent of it and without your daughter making you aware of the extent of it, then imo it's risky to continue on your own because the wrong words at this point could lead to a total and perhaps permanent estrangement.
Hopefully, this ends at it being a soon corrected misunderstanding but otherwise, this is the most important time for Mother Wisdom. The next step could be crucial so as mentioned, I'd tread lightly.
Best wishes to you. Please keep us posted.