I'm completely put off by direct incest and don't get the appeal, but I agree seeing landlady everywhere is a bit weird, especially since in practice the characters always behave in very familiar terms, which makes the whole "landlady" thing just come across as out of place. Step mom or dad's girlfriend as another user suggests seem much better imo, and preferably the dad should be an asshole, which he often is.
I like to imagine it is a fairly recent arrangement too. Shagging a person which has been your stepmom since you've been 15 years old has a completely different vibe than if she's been your actual mom since you were a toddler... which is a bit gross, but whatever. Discovering on rule34video that dogs and horses are a thing for some people kind of puts things into perspective.
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PS: Now, shagging your aunt, that's just an objectively chad move in my book, all moms should have a bunch of hot sisters which grow up to be thirsty dominatrix milfs
. Wanna achieve world peace? Just have every kid get his hips destroyed on his 18th anniversary by a bouncing hot aunt which hasn't had sex in over a year.
John Lennon has nothing on me, I'm the real pacifist.