I agree, but I almost kind of feel like you bring it on yourself in that situation.
The first time you have to choose between your cousin and your blatant childhood friend love interest, your cousin kind of acts like a shit and your friend clearly wants to be alone with you. If you go for both anyway, your cousin is clearly being aggressively sexual and moving in on your girl, while continuing to be a bit of an ass.
Then, when the actual threesome happens, your cousin is clearly showing ulterior motives. It's pretty obvious she's way more interested in the other girl than she is in you. At which point warning bells should be going off for you.
If you actually care about relationships rather than just getting laid in every possible way, you should probably be telling your cousin to get lost in both scenarios.
If you still go through with the threesome, you probably deserve to get cucked.
Though you can still be a mega-chad if you want - let the two of them hook up, and just screw both of their mothers to retain your dominance.
There really isn't a dedicated sub or dom route for the sister - it's mostly just that, when scenes with her come up, you can kind of let her take control and dom or you can put her in her place and make her sub. So it's pretty easy to unlock both sets of scenes in the gallery just by picking one choice, watching the scene, then going back to pick the other choice.
With the mom the split is early on, and once you're on one you pretty much stay on it. You can see which route you're following judging by which scenes you're unlocking in the gallery as you go. If you get the Night Drink scenes (and later the Mom Sex Love scene) you're on the love route, while you'll get the Night Games scenes (and the Mom Sex Corr scene) if you're on the corruption route.
There's not a huge difference between the two, though - even on the corruption route you're very loving with her, while on the love route you're still having sex.
TL;DR: The following is My opinion & as I haven't even finished the current content there amy be reasons for the issues I mention that I haven't gotten to yet. Also nothing said below is meant in a negative manner as I LOVE this game!
SPOILERS BELOW!!!!!!!!
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Gosh I'm super glad I read thru the last few pages bc beginning with the MC/M/S 3-way scene I'd noticed ...~idk how to call them~ inconsistencies or out of character dialogue lines. The first was how dominant Sis was being towards mom, I thought to myself *if I'm training her to be a good little sub, why is she acting like the boss over mom; Especially after the scene in which mc specifically comments on how the power dynamic is shifting? now I get it, the +Sub/+Dom in all her choices are how she interacts with the mc Only which makes sense, she wouldn't be submissive to everyone, that's not her, but it was slightly wierd not seeing her retain Any sub qualities.....especially with mom...tho I get she's trying to help her feel ok with feeling hedonistic and probably also ensuring mom knows she won't surrender any status in the dynamic, it feels just a *Touch* disrespectful, tho it doesn't bother me that much, just an observation. Also I don't want her to go full sub...it wouldn't be as fun as knowing there's a hellcat inside her and you're partially taming it, that's the appeal of getting specifically her to be more submissive *oh I've tamed a wild tigress just enough to fuck her, even tho i could still die violently*
Also regarding how many days Mc used logic, reason and love in attempt to convince mom everything was OK, then sis speaks to her for 6 minutes and all the sudden not only is she ok with it but miraculously no longer needs to go to the therapist after 29 words from sister? About that:
(Not really lmao) although I am EXTREMELY curious as to what sis said that finally convinced her....?...... did the dev have something In mind or was it just a 'sis got her to do it' thing and we'll never get to know? How evil lol!
The only other issue I have is all 3 or 4 times mc & sis have fucked her never asked her b4 hand if he could cum in her, he either just did it or told her he was as he did it and she seemed a bit unhappy the 1st time but let's be real the 2nd was fake anger, the 3rd & last she almost seemed not to care....now just bc mommy is there he's going to be a good boy and ask? Ok I understand there are/ could be many reasons for this but up to this point none is given so as readers we'd have to ASSuME? He either a) doesn't want mom to know how freaky him and sis can't get b) feels a bit unsure since Both are present but I feel this seems to go backwards in his character development. Either way bc creampies are one of my Kinks I feel we should've had the option as it could've added another dynamic to the 3 way. Unless there's a reason for this which I haven't gotten to yet?
I hope the dev has plans for it to continue on for a while, Ive seen so many games end right after the main drama gets resolved but so many of these left open or didn't explore side kinks/mini dramas and new sexual thrills. Good luck & best wishes to dev !