I'll stipulate that this is possible. I will however suggest that it is not a good idea, for a few reasons.
First and foremost, saving for college is a serious thing and it has a major impact on the child's, and their parents', lives. The child will likely make choices with their life based on, among other things, how much money they expect to have saved for college, which can begin years before college itself. The parents will be making choices - how much to save, how to save it - also years and years before.
And while you might think of surprising them in order to avoid them making more selfish choices - if they know you're good for $75k (let's say) then that's 3 more European vacations they can take, right? - it may well work the other way around. A parent who knows they can save at most $50k for college may simply tell their child to plan for state school and be done with it. Knowing there's another $75k around might mean that they save more, because they now know they could target a better school. Having full knowledge of a situation is nearly always better than having partial knowledge. If you are worried about your sibling making different and poorer choices knowing about the gift, put conditions on the gift (in which case you can't use a 529 I don't believe, but that might be the choice here).
Second, if you're going to actually give the money to the child, you should do so as early as possible in order to avoid risks related to bankruptcy or lawsuits against you. Money you've saved in a 529 account for yourself will still be reachable in most cases by creditors - while I'm sure you hope to never have to worry about that, you never know what might happen with your life. So establish the 529 account in their name, not in yours. Once it's in their name it's safe against creditors to you OR your sibling - it's the property of the child, who should be fairly safe from creditors and lawsuits in most cases until they reach their majority.
Third, if you are going to establish it in the child's name, then you need to talk to the parents and tell them you're doing it. Not like "it's required by law", but you need their SSN - and if you're thinking of finding that out with a clever ruse, stop now; this isn't engagement ring sizing. If you do find out the SSN without letting your sibling know, you risk them thinking you're trying to steal the child's identity if they do find out.