You are right that in many mangas the princes or princesses are forced to marry for convenience (for economic, social(or political benefits (or to prevent a war, becoming a bigger empire), but that stem from a historical reason , as in European medieval kingdoms or ancient empires it was a common practice ... And why except the eldest? the eldest son is the successor, that's who usually always marry for convenience, especially to form alliances between countries.
They also could be dead... or in touble. But yeah, it's more probable that they are married.
i can't believe sanji is getting married and i am guessing he has two older sisters becaus if he had two older brothers one of them could just marrie pudding