While your explanation is reasonable from a logical perspective (and I did wonder if the family connection was the case after doing my tests and noticing that the other lesbian routes were still viable, even if Ellie and Patricia were not) I think this is a case where logic is getting in the way of good gameplay. And while the concept of needing to be more filthy in order to cross the lines with family members makes sense logically, the logic in requiring male interactions does not. The message you're basically sending here is that Sophia needs to be a cock hungry slut who flirts with every male she comes into contact with in order to consider a relationship with one or two women who are in her family. That is not logical. Look at the post directly above your one where I did a bunch of tests, you literally have to choose almost every Filthy male interaction to scrape through for Ellie's crucial late game choices. At the very least if filthy points were required I think you should get enough from Patricia's various goading in order to at least unlock certain scenes, such as the gym shower one and the teasing at the park, but it's not. And it snowballs quickly.
The overall point is that perhaps I was mistaken, but you painted this game as being one with a lot of freedom and paths and options to pursue, but it's abundantly clear that's not the case. I did the tests, you basically only have three routes: Pure Good Wife, Complete Filthy Wife and Sam+Dylan Filthy. Lesbian would be a possible forth, but you cut two of the options out on that path, and if you want Ellie and/or Patricia you have to pretty much go down the Filthy Route. You may consider these limitations realism, but I play a game for fun, and to me it just makes it frustrating and the opposite of fun, because the game makes it look like there are viable options ahead of you but then it just trolls you and whips the carpet out from under you, and the whole thing snowballs so quickly because it denies you so many chances and choices that it makes those options impossible.
You could probably say that this game isn't for me, but the fact is that that it really was. It was head-and-shoulders my favourite VN on these forums for a long time because I loved the slow build up, how things played out, the characters, etc. but it was just in the last few updates that these steep restrictions frustrated me and then I went back and discovered content was just disappearing on certain paths.
To me this isn't just taste, it's poorly designed. I'm sorry, but it just is IMO. A good visual novel (or even a good RPG with romancable characters) doesn't put options in front of you and then deny them to you because you didn't pursue other characters. Like I said in an earlier post, this would be like if Dragon Age: Origins didn't let you culminate your romance with Leliana because you weren't trying to fuck Allister, Zevran and Morrigan too. The progression system here is bad because you have characters that are set up as potential romantic partners, and you can make all the right moves with them, but because you didn't do a bunch of things not even related to them directly you lose out on content and they become impossible to pursue. I don't recall playing any other VN's here that have this bizarre progression behind them. All the good ones reward you for making the right choices with the character you're pursuing and do so via interactions with that character, not with arbitrarily different ones. They don't cut you off and deny content because you didn't flirt with another character. In fact, some of them will penalize you for getting friendly with another character.
I mean, you yourself say "In the fitting room scene at day 13 you can get the lesbian point even if you don't have enough filthy points to answer Ellie on her underwear question, what just costs you an Ellie point and not a lesbian temptation point!" and that seems to miss the point. Yes, you can get the lesbian point. But what you cannot get are the three required Ellie points in order to unlock the seemingly key scene the next morning. So at this point it looks like Ellie is no longer an option if you miss that. It's also frustrating because it's so sudden that you hit the wall. Almost all the Ellie interactions up until then have no limitations and are just choices you can make that give you Ellie points. You can even rub lotion on her back and even butt and it doesn't cost a thing to do. So you have the odd Ellie moment for a while, coasting along fine, and handily reach 7 Ellie points with no barriers. And suddenly... BAMM!!! You're thrown this 21 Filthy requirement out of seemingly nowhere. For a scene where you don't even touch her. It's jarring as hell! It also seems to suggest that the content of the scene isn't as crucial as where it's placed in the game chronologically (i.e. the scene happens later, so therefore it is automatically more lewd?). I get that the scene is perhaps more suggestive in the banter than the pool scene, but it's not a whole 21 points vs. 0 more lewd.
Anyway I'm rambling... and I'm probably wasting my time. It would probably take a lot of work for you to alter things at this stage, and you probably don't want to anyway. But that's how I feel about the game. There was a lot of potential here and I like the overall style and characters. But I just feel that the progression system isn't very good and is pretty counter-intuitive, and it gives players the illusion that some paths and characters are possible that simply aren't. If I was going to offer a solution it would be to either massively reduce the Filthy requirements in certain scenes or allow Lesbian Temptation and perhaps even character points to add to the overall required total (i.e. perhaps instead of needing 21 Filthy points you simply need 21 points overall that can pool from Filthy, Lesbian and Ellie).
Overall I just think it's a shame that a game with such promise it let down by a bad progression system like this.