Unfortunately, your in-laws are people with their heads so up their own arses, that they do not see that their actions have consequences. Your DD is hurt at being excluded and not treated like a family member. That is a big deal to a young child.
I don't think there is a lot you can do about it TBH as your DH is a wet blanket. He should take his DB aside and tell him, "DB she's my DD now. I'm her dad, and it is brutal not to include her in the wedding. It is going to cause WW3 in my marriage". If he's not close enough to say this, he shouldn't be a best man.
I think you have no other option but to send your DD and your DH. I'd then take my DD out to do something amazing, just the 2 of you, for the whole weekend,. out your family pot.
Your in-laws have shown you that they are lacking in empathy and compassion for the people around them. They are unable, or unwilling to think through the consequences of their actions on other's feelings. They have told you that your DD is not family.
What I would be doing from now on is not putting your eldest DD in the way of any of them. Any DC your BIL has, are not your concern. His mum, BIL and new SIL should be of no concern to you now. Don't accept any invitations and leave it all to your DH. He may take your youngest over there a bit, but I bet they miss out on a lot because you will no longer be facilitating it.