Your mother is feeling very hurt, and she wants to do what she wishes because of her hurt and anger, and perhaps she can only justify doing what she wants by claiming it is God’s will.
Perhaps her brother-in-law may see her as a disturbing influence if she is prone to acting-out with her emotions? Her description of what she says will happen is an indication that your mother has unresolved difficulties.
We go to a wedding to show love and support, and to share in and expresses our joy in the couple’s happiness, not to bring discord and hurt to others.
Your mother wouldn’t be the only person her projects her own will to God so that she can do what she wishes. She has confused Her will with God’s but her will in this case is against Christian love, just as there is a possible lack of godly charity in the failure to invite her. To force oneself on others and be willing to cause anger to the groom and upset to the wedding guests…there’s nothing of God’s will in doing that.
I guess you understand that the rejection by her brother-in-law is eating her up and for her to project her desires on to God is within her mind a justification. It isn’t.
God desires love, unity, and peace. That is His will.
I’m sorry your mother is hurt and angry, but you don’t need to buy into her anger and her choices. It would be my impression that she wishes you to go to reinforce her decisions made of hurt and anger, as a further justification.
I guess you can understand the hurt and anger that is motivating what she says about Jesus and about her plans
I do hope your poor mother can find peace of mind.