Hi W.,
My brother's ex was very dificult, too. After they separated, she moved out of state. He had custody of their son (then 10), she had custody of their daughter (then 5). When they met to drop off and pick up the kids after visitation, she would show up whenever she got around to it, sometimes an hour or more late without calling. If he got stuck in traffic, he would call to let her know and she would tell him that if he was late, she would call they police.
If it's too late to amend the divorce papers, you might suggest to your brother that he visit a different church each time he has visitation for a few of his visitation weekends (until she gets tired of going to his church and him not being there). Then, he could go back to his regular church. If she does show up, get up and move or sit in a place that she can't sit next to them.
My brother still has his son, who is now in college, and his daughter is coming to live with him in the fall. The obnoxious parent doesn't always win!
Good luck!