You have my sympathy, OP. My brother married a woman he seemed to hate. I spoke with him beforehand. They are still married, 10 years later, and they still hate each other. Its awful but my conversation with him didn't matter, although he does remember it.
My sister, after a divorce, quickly married a nice guy, kind of a dud. I didn't say anything and while I suspect she's never been head-over-heels in love with him they seem to get along well and he is really lovely with the children. I think if I had said something beforehand, she would have remembered and resented it.