Ooh... Be brutally honest with yourself. Is your brother the kind of rational, open-minded person who can hear criticism about (and consider advice regarding) his personal life? Or would he resent the advice-giver (and always accuse you of "not loving/accepting" his wife)? Would he dismiss a family lawyer/financial advisor's advice as "too cynical" and "that will never happen to my marriage"?
IME, most people (who have not been burned already) are just not open to hearing and accepting advice like this. You really risk damaging your relationship with them both. You can try, but don't be too down on yourself if he doesn't listen to a word you say. (If you do go for the conversation, I would perhaps approach it with "unless there are prior, enforceable written agreements in place, the law may not recognize any financial debts between spouses", or something along those lines. Also, head-scratch, how would the fiancée pay him back if she's not likely to get a job?)
His view of marriage may be different than yours (that they really become "one", everything he has becomes hers, even if she initiates a divorce and walks away with half his assets and a chunk of his income. That risk is just part of the deal.) And truly, everything could work out great. You might get to see them smooch and hold hands at their 60th wedding anniversary party.