ANSWER - I see no reason at all not to at the very least consider that your daughter is making contact with your (I'm presuming) deceased aunt. If you consider the fact that most spirit encounters are made with ancestors or family members and young people/children are generally more susceptible to these encounters than adults then it seems plausible to my mind that your great aunt has chosen to visit her niece. Factor in the fact that we know the aunt to have been a practising witch and the balance of probability swings toward a genuine spirit encounter.
I'm not a fan of trying to prove this sort of thing scientifically but it would be interesting to know if your daughter has ever heard you or anyone refer to the aunt by the name "Lemon". Was this a common nickname for her or just a special name known to a few? Would the aunt have been likely to refer to herself as Lemon..?
If the encounter is genuine then my suggestion would be that you consider what wisdom the aunt may wish to impart to the girl? Has your daughter shown any other signs of being called to the craft? Might the aunt have a message for you but chosen to go through the daughter as she has more developed medium skills than yourself? Without labouring the point with your daughter or making her feel intimidated I would question her gently on what she has seen and see if you can get some more information about what she is experiencing. Don't push it though - children tend to take spirit encounters very naturally in their stride but fear can set in if the adult gives them the idea that there is something to be frightened of. Let her tell you at her own pace and keep the subject light. There is after all nothing of concern here, if the encounter is genuine it is your much loved aunt making contact with her family. What could be more benign?
Of course we do have to consider that there may be a rational explanation that has nothing to do with the spirit world. I get very frustrated with witches who are eager only to view one side of the coin and look for a magical/spirit related solution to everything. Like it or now we live in a world where magic and the mundane blend together. Not everything has a practical explanation and not everything has an other world related explanation. As witches we have a duty to consider both the magical and the mundane before forming any sort of conclusion. In this case it is perfectly possible that your daughter has heard the name Lemon mentioned and it has captured her imagination. She could be creating a fantasy (children are very adept at blending fantasy with the real world) or she could be deliberately feigning a spirit encounter to get attention. I actually think the latter is extremely unlikely - a twelve year old has much better ways of getting your attention and would be unlikely to grasp how much a fabricated tale of seeing her dead aunt might disconcert you. For my money I'd say your daughter and your aunt have made contact and that the best thing you can do it sit back and wait to see if either the aunt has a message for you or if your daughter shows an aptitude or interest in witchcraft that needs to be fostered.