Now, I know that most of them are the youngest, but there are a number with the oldest. It's the oldest of the twelve dancing princesses that marries the soldier. In "Fair, Brown, and Trembling," Brown's only place is to make Trembling the third; it's Fair who acts as both the stepmother and the stepsister in this Cinderella-like tale. There's all the tales where the stepchild has a half, not a step, sibling. and there's "The Greek Princess and the Young Gardener" where the hero is indeed the youngest, but when the talking fox asked to have its head cut off, the oldest one steps up to do it for him. . . .
Which is more than most middle sons or daughters do. There's "One-Eye, Two-Eyes, and Three-Eyes", where the middle daughter is the Cinderella figure because she has two eyes, like ordinary people. And then there's a Greek form of the Singing Bone. Yes, the youngest is the admirable one whom the eldest envies and murders, but the singing explicitly proclaims that the middle one is innocent as well as that the eldest is guilty, since they were all hunting a monster boar at the time. By fairy tale standards that is a lot.
I think I may have to fudge something together. There's a trope where three princesses are given three fruits. Like the eldest gets a ripe one, the next one just starting to ripen, and the third one that's green. Or the eldest gets an overripe one, the second a ripe one, and the youngest one that starting to ripen. Which is a metaphorical indicator of ages. In the first, the eldest is ready to marry, the middle approaching marriageability, and the youngest far too young. (Naturally, the hero marries the eldest.) In the second, the eldest should have long been married, the second is fully ready for marriage, and even the youngest is just about ready. (In that case, the hero marries the youngest, who gets to pick like her sisters even with the "not quite" element.) Push 'em together so the youngest is not married off, and the middle one could walk off with the hero. . . .