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.
That was a great story intro. She took his words seriously (to go home first and come over to play) and he held to his promise literally. And how he grabbed her on pg 79 was just plain cute as hell.
It could've been a choose all the correct answers and not just a simple A/B/C/D. There could have been A&B&C&D could have been a correct answer. You don't know what the correct answers are so it's still possible.