it’s very painful for an older sibling to watch a younger one get married before them. so it’s really impossible for anyone else to judge whether or not it’s right. in addition, i think this is one of those things that goes according to the family’s custom, so again, no one can judge.
that having been said, i think it’s not such a good thing. i would allow my younger siblings to go before if it came to that, c”v. it’s a chaval for all the other siblings to be held up just cuz the first one hasn’t found his/her bashert yet!
i actually have a teacher who got married at thirty. before she even met her husband, she MADE her little sister’s shidduch!! now that’s an extreme, but i thought it was a good story. 🙂