I think the first thing you need to accept is that it's none of your business why his marriage fell apart. His wife may be a saint, but relationships are complicated. Clearly, he's not in love with that saint anymore.
It's unfortunate that her financial and emotional life is not going well - you can certainly assist her with child care or whatever else. But blaming your brother for not maintaining an unhappy relationship with her isn't helpful to her, you, or him. Sometimes people break up. Sometimes nice people break up.
That said, what he's doing to his children by not being involved with them is unfair. If you maintain a friendship with your brother, you might be able to influence him to do more for them and with them. It's possible that once the excitement of his "new life" dies down, he'll decide to become more involved. Or, he might not. If that happens, you'll need to decide if your brother is someone you care to have in your life.
Keep in mind, that you do not need to make any drastic decisions. You can let the relationship fade by politely declining invitations, exiting conversations, not giving him nice birthday gifts, etc.
I suggest that you wait and see. Their divorce isn't even final yet. It will take them time to learn how to co-parent in a divorce, if that's what they end up doing.