Sister Trap definitely lives up to the plot synopsis - at least in the first 7 chapters. The story is quite stunted, and almost exclusively revolves around this struggle that the MC has with his girlfriend's sisters. It's an amusing setting, but there's no plot development.
The art is well done and captures the scenes quite nicely. I've seen better, but this one is nothing to sneeze at. It's definitely the most redeeming quality of the series.
The characters are a bit flimsy and one-dimensional, but at the same time they feel moderately believeable. The Girlfriend is the weakest character of them all, but in her defense the story wouldn't exist if she didn't behave the way she does. The sisters are just quirky enough to feel like 2 individual characters, but that's about as far as it goes: everything else is just "We are horny and want to seduce you away from marrying our sister". The MC is the most developed character, which is a bit of a pleasant surprise.
Overall, I'd say it's a hair above average for a manga of this nature, and that's almost entirely due to the art quality and clear representation of who is doing what.
I daresay, though, that there's enough gratuitous and explicit content, with almost nothing else to pad it out, that it could probably be moved from the "Borderline H" category to just straight-up Hentai.
Eh. Outside the first 5 chapters, it's a bunch of one-shot stories that are boring(except for the first one-shot). There's no character development or interesting characters in general here. It's pretty much a hentai manga.
It's somewhat funny and somewhat sexy.
At first Chisato's sisters are trying to seduce Naoto to just get fem out of Chisato's life, but it quickly turns into them trying to steal fem for themselves. And it's not subtle in the slightest. They are basically molesting and raping fem, as they have apparently done with all of Chisato's boyfriends. This is of course remorseless, predatory behavior. And I know it's probably difficult for Chisato to call the police on feir own family, but they are engaging in criminal activity and are repeat offenders at that, so that's exactly what fe should be doing. At the very least, feir sisters need professional help with their mental problems. And Chisato should probably cut them out of feir life, because they don't love fem in a healthy way and are going to continue hurting fem and feir relationships. That being said, Chisato was right to punch Naoto in the face. If only fe could act similarly to feir sisters and do what needs to be done.
After the "Sister Trap" story, there are three other single-chapter stories, tied together only by a shared sexual theme. The "Her Boyfriend" story felt like it would win the Most Likely To Actually Happen award. The "Pure Girl" story was kinda funny, though I was left wondering why Shin assumed that fe had great technique if fe's only had a single partner. The final chapter is probably the worst, both storywise and sex-scene wise.