I don't know about your brother, but I would definitely advise his fiancée to get married before they have kids, if that's on the horizon. With such a significant income gap, there's going to be pressure on her to be the one to do prioritize childcare over developing her own career. Even if she doesn't quit work to stay at home full time, someone has to take care of the kid who's kid is too sick to go to daycare, do the daily drop-off and pick-up, etc. In my observation, in couples with big income gaps, the lower-earning parent almost always takes on a disproportionate share of that labor. Which is an entirely reasonable thing to do! The reason it's so common is because it's usually the financially optimal choice. But if they aren't married, it means that the lower earner doesn't have much recourse in the case of divorce.
If they don't have kids, marriage may be less important, although they should still make sure they are on the same page in terms of who is on the deed to the house (if they buy one), funding for their respective retirement accounts, etc. I'm sure your brother is a nice guy, but I have seen so many people--usually women--get screwed in this kind of situation.