I’ve spent a couple of days looking into this
I’m sorry to actually bring some facts to the CR but if you are interested here goes.
There is definitely an Inyan in waiting and maybe even for asking mechila/permission from an older sibling.
It seems to stem from a tosfos in kiddushin that says that since Yaakov had no response to Lavan we see Lavan had a point. There are several places in Shulchan Aruch that address the inyan in regards to a case of; a man saying he has married one of two sisters but doesn’t remember which, do both need a get or only the older (since the way of the world is to marry the older first)
There is even a couple of modern tshuvos on the matter.
But
No where do I see that this is a minhag (2 years) or that this can (or should) be forced on the younger sibling.
It seems that the younger sib owes this as a courtesy ie derech eretz (and shalom bayis within the family)
To quote one tshuva “use your sechal”