From what I'm getting, if your brother didn't time skip, then your brother is remembered. Dead or alive. There are probably a lot of people who were at Carteneau and didn't get skipped ahead for reasons and obviously Eorzea remembers the event happened so not everyone could have been skipped ahead.
Would they forget the younger brother being the older brother? Probably. The younger brother would be wiped from most memory. The confusion would come from them thinking the older brother was at Carteneau but they have no idea why they think that, they just do.
Would they forget the younger brother being the older brother? Probably. The younger brother would be wiped from most memory. The confusion would come from them thinking the older brother was at Carteneau but they have no idea why they think that, they just do.