It frustrated me too, more so because it was so downplayed. And I feel like Isabel was just kind of inserted into that position. Like, I know she's a villain, but he could've added a really cool character, or their dad, but I'm not a huge fan of I'm-alive-surprise-surprise moments (unless that happens in Mockingjay Part Two to a certain character- you'll understand when you finish the series). I always just thought that Arthur used the nickname in the presence of the family and Amy just had forgotten about it since she was so young at the time. So I think that's how Isabel knew, but I really wish he'd used a different character to be his dad.
Yeah, it honestly felt like, "Oh, well, we don't want to have Arthur alive again because that would cause too much complication, so let's just shove Isabel in there." Personally, I thought it would have been super cool for Arthur to not have been dead. Ahhhhh I need to read Mockingjay...Well, Amy knew about it- she told the story to Dan, right? Or am I just being blonde? Haha, that's always an option. Isabel must have heard it at some point, but I would have thought that the family wouldn't have used it around her.OH!!! THE HOUSE WAS BUGGED, REMEMBER?? MAYBE THAT'S HOW.