I get why you feel that way. But if your mother in law was anything like my own mother it would cause more drama if I didn't say why I wasn't helping.
Your mother in law is asking, not your BIL or SIL. Don't associate with them. I get it. But your MIL doesn't know what happened and just sees her son saying no to a simple request and that's probably stressing your fiancé out with the guilt trip he's getting from her. It's not technically helping them- it's helping MiL and family who are uninvolved with your disgusting BIL.
Is the wedding discount the same as the one your fiancé can offer? He can lie and say he only has so many discounts he can offer a month or something?