I see..

I think I agree with you. But you have to sit down your wife and be very respectful and kind to her. Tell her that you support her, and that you are not diminishing her feelings, but remind her that other people's actions shouldn't affect the relationship with your brother. Tell her that you love her and your daughter, and that if this was (insert names of the family members who were shitty) you wouldn't even be considering going to this wedding. But this is your brother, someone who wants both of you there, someone who's been nothing but supportive. That at the very least, you have to go and will completely understand if she does not want to attend (with fair reason).

I personally think that her going with you is actually a power move on her part. Shows balls and character, and more importantly, class. Because she did nothing wrong. She would be there to support you, supporting your brother. Nothing more, nothing less. Give her options to show up and leave, or have a safe word for when she's ready to get out (if she ends up going) and also have a serious conversation with your brother and fiancée and tell him that you will attend with the warning of dipping out at any minute she feels shaded by the family. If he is as cool as you think he is, he'll understand. Regarding the rest of the family, I wouldn't even give them a heads up. I'd show up with your wife, hand in hand, wearing her best fucking dress, with fancy jewelry, and not caring about their shitty opinion.

Good luck, OP!