I'm thinking in Renpy terms, but this should work in other cases as well. It goes along with the 'form' idea that you mentioned.
You'd create a variable that would add a descriptive word, that you'd use in this context. An example would be 'miss' as the default variable, but you could change it to 'mom'. Then write the story as if 'mom' were there, but the variable would replace 'mom' with 'miss'.
Similarly, 'roomie' could be the 'default' where you actually want 'sis'. I'm sure there's a more useful term you could use here, but I think you get the idea.
The variable name itself could be something short, like 'ls1'. You know that ls1 stands for little sister 1, but ls1 by itself could mean anything, should Patreon dive into your code looking for key terms or something.
You could also have 'ls1' and 'ls2' as descriptors for the same person, to allow for different terms of endearment. I.E. 'sis', 'lil lis', etc., which would replace say 'roomie' and 'girlie'. I suck at substitute terms, but you get the idea...
The intro dialgoue and such could also be tied to a variable. In this case, you'd have the 'Patreon safe' intro dialogue tied to a specific variable(s), but then there'd be a patch somewhere other than on Patreon that would change the text entry for said variables. You probably won't want players filling in these 'story' variables though. That'd be more suited to a patch, but I suppose if someone wanted to create an editable 'backstory' variable, so that someone could say create their own background, i.e. 'your a Martian that recently adopted human form blah blah'... well the rest of the story might not make sense with that background, but yeah...
This may mean that your story makes less sense with the default variables, at least as far as your intent for the story, but if you think creatively you can still make this work well enough. Just remember that most people are probably just playing your porn game for the fappable moments anyways... the story is just for tittilation purposes, it's just a steppingstone to get to the hot sexy renders that they want to see!
As for the 'patch' idea that you've seen elsewhere on this forum I'm guessing, well applying the patch would change the default variables to 'mom' 'sis' etc. for players installing the patch, so that players wouldn't have to guess at these themselves. Just don't share anything about the patch on your Patreon account, and see if you can get an anon to handle the patch for you, so that your own Patreon username isn't associated with the patch...
As you can see, it requires a little more forethought as to doing the scripting, but once you map out the descriptors in advance, it's pretty easy to just include the descriptor variables, similar to how you use name variables, in the dialogue. At least with Renpy.
I'm sure that RPGM and other engines have similar mechanics, but I've never messed with those. The concept is the same though!
BTW, a good example of where a developer had to 'bend' the story to meet the new patreon guidelines is Dreams of Desire. Mom became Mrs Turner in the 'rewrite', but some anon made a patch available that changed everything back of course. Might be worth a look.
