Hello, I'd like to help you with your question.
You are not nuts at all. And you have every right to be hurt by your wife's actions. This is, after all, your first wife's family. So the relationship between you and your first wife's family is through you. Although is it very kind of your first wife's family to include your second wife in the family gatherings, it is still about you and your first wife.
If your first wife's mother has stopped including you in these gatherings and will not tell you why, that is unfair to you. She should at least let you know why she feels as she does so you can either try to address the issue, or know why you need to stay away.
The fact that your first wife's family still includes your second wife in their invitations seems to be a direct message to you in order to hurt you more deeply. And that can cause enough pain in itself. But the fact that your wife ignores your hurt and still attends the gatherings only adds
to the pain. She is basically saying that she sides with your first wife's family and not with you, when her loyalty needs to be with you.
It is not unreasonable to tell your wife that you are hurt by her actions. She is going against you by ignoring your needs. She should at least talk to you about the invitation and ask you if it is ok if she goes. And because of her friendliness with your first wife's family, she could even offer to try to mend the issues between you all and see if she can help you rather than go against you.
Try again to talk to your wife about your feelings. If she chooses not to listen, ask her to go to therapy with you. This conflict has the potential to drive a wedge into your marriage and cause a larger issue. But if she will not go to therapy with you, go on your own. You need the support and the opportunity to work this out and decide how you want to address it.
Also, try to talk to a member of your first wife's family that you feel you might have a closer relationship to, such as a sibling or cousin. They might be more open to talking to you and helping you find out why the mother has stopped talking to you. And if you can repair the relationship, then this issue may resolve itself.
I hope this has helped you,