The matter that people have with this isn't that it's incest (which obviously it isn't a guardian doesn't have to be a mother) but it's the age thing and also the fact that she raised him. She raises him from the age he is basically afraid of the dark while she is a full-blown adult and then they are involved romantically....that doesn't sit well with a lot of people.
I understand why they're bothered. I'm not bothered personally because I read the most messed up shit that this is nothing at all but I understand why it might be a deal breaker for some readers
Same i often forgot that i myself had read some of the craziest shit and this thing doesnt bother me at all. And the witches and a child arts really did me good. I love those arts altho it was just a short 2 pages or 4. But yes people, witches do be like this. Stay for the resf of the plot or leave if you think its not right
I personally do not mind the whole "I have a thing for my mother figure" sort of relationship, especially if A: the figure is really young/young looking and B: they're a mythical creature. Mainly in these types of stories, it is the younger person that breaks the relationship of parent and child.. I can see why some people would be put off by it.
There are a lot of people who claim that this type of relationship is grooming, but that's inaccurate considering that she isn't raising him with the intention that he will be a sexual partner for herself.
Also, author is aware of this because (spoiler) a big part of the drama in this is her seeing him as a man/romantically literally because of the fact that she practically raised him.
Imo, shes just a guardian not a mother. Ive seen a lot of witches taking care of a child fanart and theyd end up being together altho some died since the witch lives forever/long life and in this case, age doesnt matter at all. A common theme, a witch and a child they raised, not bad and not a red flag for me (●'◡'●)ﾉ