Dude...breathe.
Go to the wedding. DJ the wedding. Eat the food. Smile at people. Wear a fake moustache if it makes you feel better but GO. You already made a commitment to go. Kill em with kindness. If the brother says anything to you, smile nicely at him and calmly let him know that you and his sister worked that issue out long ago and its all resolved but that you understand his concern that you would feel the same way in his shoes. People eat up empathy like candy.
However, if you really want to make it all better call your ex up before the wedding and offer to add, say, $20 a month to the child support you currently pay until all the unpaid back child support is paid off. It takes a long time, but $20 a month isn't going to put you in the poorhouse and it demonstrates a genuine concern to make things better. If the ex wants to accept the DJing as back payment that would work as well. Either way you are going to be the LAST thing on her family's mind that day so I don't really thing you have anything to worry about.
There are very few personal problems that cannot be solved with a suitable application of high explosives.