You can expect their definition to keep changing as developers find ways around their arbitrary rules. Safest way, I suppose, is to allow the player to define relationships in the game or just have all the characters exist as friends and issue 'unofficial' patches, posing as a fan!
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I'm trying to figure a game working well with that premise, maybe using some modern AI subroutine
G: "And here is a milf living in your own house with you, you call her....[ENTER HER ROLE IN THE STORY]"
P: "Landlady"
G: "Erm, I've been programmed exactly to avoid the landlady trick, so don't worry about hiding your true wishes. The hot milf is your... [ENTER HER ROLE IN THE STORY]"
P: "Landlady"
G: "No, really, can't you think of something hotter and more taboo (wink wink)?"
P: "No"
G: "Come on, it starts with M and is a three letters word palindrome"
P: "What is a palindrome already? "
G: "Dammit! Just type MOM when I ask again about her role! So, what role has this hot milf living with you? [ENTER HER ROLE IN THE STORY]"
P: "Landlady"
G: "WTF? Are you stupid or what?"
P: "I have a fetish for the landladies, I've read there was a landlady in the description, this is the only reason I've subscribed!"
G: "A fetish for the landladies????"
P: "Yeah, and you scammed me with your game without a proper landlady, I'm going to report you to Patreon"
Jokes aside, I fear that a game where the player can define the relationship either will have plot and dialogue horribly generic and dull or will sound incredibly stupid and out of context if someone doesn't choose "MOM" (stupid in the way that the daughter of the landlady -aka sister- says can't have sex with the MC because "we live together" or something on that line)