It will be a lot longer than that. Currently those that applied 18ish years ago are eligible for visas, but that category has only moved forward 3 years in the last 10. Unless things speed up considerably (unlikely, given more and more people are joining the line) then it’ll be a lot longer than 18 years for those that apply now.
So it doesn’t help your nephew at all, as he will have aged out by the time your sibling is eligible for a visa anyway. But if your sibling is interested in moving to the US, then you may as well file for them now.
Good luck.