My brother just asked me to get ordained and marry him and his fiance at Thanksgiving, what do I say?

So here’s the story. My brother got engaged to his fiance a year ago. For some reason he wants to get the two of them legally married before the end of this year so they can “just plan and do the wedding next year.” He says it isn’t for healthcare benefits, his fiance isn’t pregnant, and neither of them are facing life/death decisions at the moment. My mother has made it clear that he had better not get married without her there when it happens even if it is before the wedding, which is I think why he’s asking about this way. He literally just asked me out of the blue yesterday to get ordained online and legally marry the two of them in 17 days….. I already reminded him that if he did this they would be required to file as a married couple which, to my knowledge, wouldn’t benefit either of them. Every time I ask any questions about it at all he gets all pissy and doesn’t answer the questions saying “everyone keeps asking a million questions saying the same things.” I just don’t understand the rush to get legally married this year, let alone to have me get ordained online do it on such short notice. Marriage in my mind is more than the legal aspect of it, it’s binding one soul to another, which honestly doesn’t even need a ceremony to happen. I’m not sure if I’m comfortable performing that binding even if it is just legally speaking. Help, what should I do?!?!
(Additional info: they live/would be married in Florida)