Oh, Yes! The junk REALLY accumulates when you have kids underfoot. Been there, done that!
I got my boys in the habit of going through their things; books, clothing, old toys, etc. on a yearly basis and I would take it all to St. Vincent’s if it was still in good repair. (And, of course, keep the donation receipts for taxes!)
Hopefully I showed them two things: How BLESSED they were to have so much stuff to begin with, and how being GENEROUS and giving to those in need was A Good Thing.
With Beau, I always equate it to a $$$ amount spent on ‘stuff’ or that he could GAIN by selling said ‘stuff’ or taking it to the scrap yard. That seems to be his hot-button motivator to get his rear in gear, LOL!
Box method maybe. If it fits you can keep it, if not it goes. I have some nice wooden toy boxes that would work.
And your sib will have less reason to come to me complaining about, "their stuff is ALL over the room! Make them clean it up!"