Is it a sin to watch Harry Potter movies and other movies about witchcraft even though you don't believe in it?
The closest scripture I could come up with would be Philippians 4:8. “Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things.” This does not, however, say that it is a sin to think on other things as well.
One of the most difficult things to teach a child is the difference between make-believe and reality. This is true of books, television, and movies. That is one reason some movies and television shows are rated for viewing by older audiences. As long as you know the difference between fantasy and reality, you will not find anything in the Bible that prohibits watching these things.
I have read and enjoyed most of the Harry Potter books and seen some of the movies. While at least one author objects to the idea that the magic done in these books occurs in the real world rather than another world, I am able to make the distinction between real and imaginary. My main objection to the Potter stories is not the magic but the morality. Rather than defeating evil by doing good, Harry (and to some extent his friends) resorts to breaking the rules. That good triumphs in the end is irrelevant; the use of wrong to achieve good is unbiblical.
I tend to draw the line, personally, at some of the horror movies. That may be because they are not that good. But it may be because they tend to be set more in “reality” than even the Harry Potter books. Because they are about things happening to ordinary people, rather than to people with special abilities, I find them more disturbing. Again, though, that is a personal point of view. The Bible is essentially silent about whether to watch such movies, if you can do so without them causing you to dwell on thoughts about sin.