To me as a child it wasn't strange at all. Later in life my ex-husband and I used to call an old sister 'auntie', she was the dearest friend of my (then) inlaws and a friend of their family for decades, and my children changed that to 'granny'. Every since I've always referred to her as 'my grandmother', even though she wasn't really related to us, it just felt that way. And she was happy with it, she never had children of her own. We were the only family she had. She's passed away over 14 years ago, but still to this day she was closer to my heart than any of my blood related grandparents.
My new husband has never been a JW, but he has numerous stories of neighbours he'd call 'uncle' and 'auntie', so I guess it's also a cultural thing in the Netherlands to give those 'titles' to people who aren't related but still somehow close to the family.