Hi, Anon!
I do not consider Catherine and Heathcliff to be incest. It’s an incestuous situation, I will certainly say that. But given how old Heathcliff was when he joined the family, I compare it more to a foster or step-sibling relationship, which doesn’t fall under my personal somewhat strict categorization of incest for this blog.
If the adoption happened after the age of 5 I don’t consider it incest. This is a pretty hard and fast rule for me. On some level it’s an arbitrary number but it is based on psychology and psychological studies. I’m not as interested in adoptive situations which is why I’m a stickler about what I count and what I don’t count.