Their relationship is explained through the narrative on the 2nd page before she ever calls him uncle. It's fairly clear she's calling him uncle as a term of endearment.
Personally, I only found it confusing because she screams it with an exclamation point. Combined with the fact that she's tossing water over herself, I thought it might be "Uncle!" as in "Mercy!" or, if you're from the American South, "Oh, lawwwdie!"