My goddaughter has for some time now jokingly said her younger sister did things that she clearly didn't. At first it was always situations where I clearly witnessed her do something and was myself jokingly asking who did something and she would say her sister did it. In these cases it was clear to everyone who did what, and that no one was in any real trouble, and it was just play, so I had no problem with it and kept playing along.
She also likes to occasionally pretend that she is her little sister, and vice versa, and thus tell me that her little sister did anything she did or that she did everything her sister did; but again it was always clearly just imaginative play.
Lately though there have been times when something 'bad' was done and she tells me her younger sister did it to avoid getting in trouble. I can tell by the fact that she preemptively rushed to tell me, the fact that she continued to insist on it even after I asked if she was just playing, and from her extremely poor poker face that she was afraid of punishment and was actively trying to avoid it by lying. I've discussed lying with her and punished her when I catch her in such a lie.
Unfortunately, the 'joke' that her little sister did something has become so common that now that she has started using it as an intentional lie she's quickly bluing the lines between joke and lie. There are times when she is so quick to say her sister did something that I don't know if she realizes I can clearly tell who was responsible and is just trying to play, or if she thinks she may be risking trouble and trying to avoid responsibility.
Obviously we need to set a limit on when she's allowed to say her sister did something and draw up some firm rules so that I'm not in doubt as to rather she is trying to play or trying to lie. I also need to explain what these rules are, and why they exist, in a way she can understand.
My question is what a reasonable rule is. Do I have to tell her that she can never say her sister did something, even when it's clearly part of play as either a joke or imaginative story telling, just to avoid any risk of her getting confused as to when it's allowed to say and when it isn't? After allowing her to playfully say that her sister did something for so long I'm afraid suddenly forbidding it in play would seem like a confusing rule change to her; plus I hate to limit her imagination play if I don't have to.
However, if I don't outright forbid playfully saying her sister did something then how do I draw the line between that and anything that may be construed as lying such that she will understand the difference and there will never be a question about what is or isn't allowed, and when I should treat something as play vs a lie?