It's just Solid—no Frills, no Feature Bloat; just a good game.
Sometimes classics are classics for a reason. Sisterly Lust is one of those cornerstone foundational AVNs that hark back to an era when renders were 2.5d renders in Poser or DAZ studios and not everybody and their brother was making adult games.
It relies fully and commits wholly to the incest kink—it's, quite literally, in the name—so people who aren't able to get on board with it will struggle with this game. It respectably has its niche and it sticks to it.
What makes this game a classic is its understanding of what it is. It's a harem porn game with a basic plot, a large variety of girls with different appearances and body types, and the inappropriate tension and sneaking around of forbidden (incestuous) relationships. And that's really it. It understands that's all it needs to be.
Unlike a bunch of games today that try to pad out the tension with some outrageous overarching plot or overwrought tension that keeps the MC from scoring with the girls, this game pares it back. It gives the characters room to breathe and allows their personality and behavior to carry the day. Because of that simple philosophy, the developer is forced to focus on the characters. They aren't innovative or exceptionally written, but they are obligated to be good enough. And that's what they are: good enough.
Lisa takes time to warm up to the idea of a sexual relationship because she's virginal and starts the story with a boyfriend. So the struggle with her is that the MC needs to teach her to explore her sexuality and then demonstrate that he can be a better man than her scumbag boyfriend (admittedly a very low bar). Bella starts off hating the MC because of abandonment issues stemming from her father, so the MC needs to either subjugate her or show her the love she never got from him.
It's not much, nor is it original or groundbreaking, but it doesn't need to be. Adult/romance/porn games already come with a massive suspension of disbelief. People who play them see a hot person and want to get with said hot person. Authors just need to be able to establish interest, build character, introduce some tension. Give the audience a reason to be invested in the storytelling of their relationship, and people will enjoy scoring with the hot girl. This game does that with no frills and nothing extra.
Most importantly, it doesn't break the immersion by trying to do too much. Characters behave reasonably, porn logic aside, and progression between them tends to flow naturally. Some of the harem are easier to get into bed and others take more convincing, as you would expect from their characters and context. But for most characters, their story doesn't just end once the MC gets them in bed, because the developer at least has an acceptable understanding of human relationships and recognizes that sex is just one part of the interaction. Even if you play a solo girl route, their stories feel rewarding because each step builds upon what's already happened.
Yes, the graphics aren't particularly good. Hell, they weren't especially good by 2018 standards either. No, there aren't any animations. Yes, it's basically a silent game. It wasn't groundbreaking back then, and it's certainly not groundbreaking now. But it understands that it doesn't need to be.
This game recognizes that the best tool for creating a harem fantasy is in the reader's mind. It engages you and allows you to enter this world with a weird MC who's attracted to his hot family and wants to establish a power fantasy harem with a dozen other girls in the mix. It establishes one central antagonist (quite literally the morality police) and just lets it sit there in the background, because that's all it nees to be: a background threat.
The world building is simple and effective. You're in some ordinary suburban family in what could be anywhere (it's never explicitly stated). It's not particularly important because it doesn't try to turn what isn't necessary to the story into something. It doesn't try to make the setting its own character, which is effective when done right, but often disorientating when unnecessary or done poorly.
It's not trying to create some dark and gritty atmosphere or elaborate fantasy world. It doesn't create some obnoxious sandbox where you need to click a million times and manually move the MC to work each day. It doesn't bait and switch you by dangling the possibility of a sex scene only to have it get cut off or interrupted through contrived twists. It simply focuses on the girls, their personalities, and the MC's pursuit of them.
In this genre, like many others, simplicity works. This visual novel embodies "Keep it simple, stupid!" and the result speaks for itself.