Honestly, what irks me most about this formula is the lack of diversity in its execution. I don't even have much of a problem with the power fantasy aspect this genre often plays up — porn games that try to introduce conflict, unless built around that conflict with an appropriate reward mechanism to match, tend to get bogged down by constant blue-balling, interruptions and unnecessary lore — but you can do more than "son with a mother, dead/absent father and two or more sisters."
You could have pairing-specific games (father/daughter, mother/son, sister/brother). Some of my favourite games do this, and they're not even particularly good, all things considered. Focus hard on two characters, make the experience entirely about getting them together, and you've got a banger. Do it well enough and the restrictive premise will make it even easier to implement meaningful choices — you're only planning out future story divergences for two characters, after all, instead of five or more. Make a multi-route narrative with intricately distinct outcomes based on a variety of small, cascading variables. Construct plot fulcrums and make them matter. Put in game mechanics and stats if you like; they'll be easier to account for when you've only got two characters to track, especially if one is the main character. The MC probably has no gated decisions, and few stats that matter where choices are concerned — it's the love interest who matters here. Love, lust, anxiety, whatever you want to quantify. Go wild. The small scale allows far greater depth.
You could go in the other direction and have more story-focused games with complex relationships, perhaps where the family is more an ensemble of equally-important players than a harem to a single MC, with twisted incestuous relationships playing out behind closed doors without other members knowing. Father molests sleeping daughter in the dead of night, causing her to dream of getting together with him. Mom finds herself around son more and more due to her husband's repression and lack of love/time for her, leading to slow, tentative romance and increasingly inappropriate touches. Remaining sibling(s) flash each other for "fun", gaining courage each time, while the rest of the family remains blissfully unaware. Have things progress gradually, coming to a head as one member (or all of them) begin to realise they're not the only ones with these perversions. Either they find it hot or become hypocrites, angry at the others for engaging in the same sins as them. They double down on their own incest as a form of twisted revenge, not telling their partner(s). It gets messy. Pregnancies happen. Lies are told. The few remaining lines are crossed. Your choices for each character could turn the ending into a bitter separation or a debaucherous family union.
Hell, if you really must go with the same old formula as in the OP post, make it so the dad's still around. Maybe the game's a netori VN where you cuck dad as a core focus of the story/mechanics. Maybe it's more adventurous than that, and the MC is given the chance to rope dad into the action as one of the LIs, feminised and buck-broken, or burly and a dominant top (lots of bisexual sandwiches in bed and trains running across the living room in this household). Maybe dad's a willing cuck, and is the one to spring the idea on his son and set him up for all the incest to come because he gets off to watching taboos from the shadows; son might not even be into it at first, and the player must prod him in the right way to get him to even think of pursuing his first family member. Maybe it's a spying/voyeurism game where you play the part of the odd one out, slowly discovering the debauchery unfolding under your nose, or that has already unfolded without you being aware of it until now. Maybe it's a mind control game where MC learns how to subtly push people into perversion using hypnosis that's closer to the real deal than usual, emphasising the eroticisim of slow corruption instead of instant personality rewrite. Maybe it's a sleep sex sim where your goal is to impregnate each member of the household without any of them waking up or catching wise to your nighttime escapades during the day, requiring careful cleanup of the scene, allaying suspicions as the plot presents them, and learning how to be a good deceiver; someone could even find out and be into it, giving you a willing accomplice to your shenanigans while everyone else remains oblivious.
If the old formula is to be used, at least put a new spin on it. Innovate a little, or do the same thing as someone else but better. Don't just make another soulless repeat of "son plus mother plus sisters plus dick equals fucking within five days." If I could make art worth a damn, I'd probably try my hand at something. Unfortunately, I've never been skilled in that department, and I have neither the time nor the patience to create a game of my own.
