Ain't it grand?
I may have mentioned this before, but if you're talking about the siblings of your grandparents, you should use the terms grandaunt and granduncle, not great-aunt and great-uncle. The same goes for the children of your nieces and nephews: they are your grandnieces and grandnephews, not your great-nieces and great-nephews.

When I was a kid I used to get confused as to why the sister of my great-grandmother was called my great-great-aunt. Why did the first have one "great" and the second have two "greats"? I thought that it would be much easier if each generation used the same number of "greats", and it turns out that I was correct. Unfortunately, it has become so common for people to use the wrong terms that some dictionaries actually seem to prefer the "greats" over the "grands". I understand that language is supposed to be fluid and should evolve, but it only makes sense to me for this to happen when the word change is something that seems logical and makes speech easier.

When going back a few generations, the easiest way to remember the correct terms and to determine the correct number of "greats" is simply to use the same number of greats for the aunts and uncles that you'd use for your direct ancestors. For example, if you're talking about the sibling of a grandparent, then you're talking about a grandaunt or granduncle. If you're talking about the sibling of a great-grandparent, then you're talking about a great-grandaunt or great-granduncle. If you're talking about the sibling of a great-great-grandparent, then you're talking about a great-great-grandaunt or great-great-granduncle. Et cetera, and so on.

It's not like you call your mother's parents your great-parents or your great-mother and great-father, right? So why would you leave the "grand" out for aunts, uncles, nieces, and nephews? The answer is, you shouldn't. The only time you should use "great" with an aunt, uncle, niece, or nephew is when that family member really is great: very large, very generous, very loving, very accomplished, very interesting, etc.

It's all quite simple, really, but if you insist on using the great-aunt/great-uncle/great-nephew/great-niece terminology, then you should be consistent and then use great-great-uncle, great-great-great-aunt, etc. for subsequent generations. But some people use the term great-aunt and then, referring to the next generation back, say great-grandaunt. If you choose to leave the "grand" off for your grandparents' siblings, then leave it off for your great-grandparents' siblings, and so on. You should say great-great-aunt, since you've decided not to follow the logical progression of "greats" and "grands". Don't leave the "grand" off for one generation and then use it for another. That is neither great nor grand.