Monkey Business is one of those AVNs that make you feel like they could have been extraordinary, the state of the art, but fail to shine as they should because this or that. You have a hell of a good time with them, yet you are left wanting the developer had gone the extra mile and made this the masterpiece it deserved to be.
In this case we have a good enough story, with some plot holes here and there but just good enough not to spoil the overall experience. Also, maaaybe after a certain point the plot takes a dive and it becomes too much of a fuckfest, like some other reviewer pointed out. Animations are decent - kinda short'ish and sketchy at times. Continuity among scenes also sometimes leaves something to be desired. There's virtually no sound, sometimes character titles are wrong, there's a significant amount of typos and English is subpar at times. I played this with the multi patch installed btw.
What does make this an outstanding adult game for me despite all those flaws is the absolutely sexy situations you find yourself in (it's the situation itself or the camera angle what gets you going more than a killer render) and how gorgeous -while still realistic- the LIs are. Mom is a red head (a plot hole in itself, since neither are you or your sister) that becomes raunchy after drinking some wine... She makes you want to bang her since the first time you ever lay your eyes on her. On the other hand your sis is your typical pushy and annoying older sister, but probably the one that better plays the part in an adult game. You get to decide if you want to follow a love (maledom) or corruption (femdom) path with her, leading to different situations. Mom gets a similar treatment. Now, the experience is satisfying as it is... but if only facial expressions were more varied, you feel things would be so much better. Yet you can't help feeling that some renders are missing, seeing as sometimes a long talk is staged with less shots than you'd expect. Or, you don't read the dialogue that you'd expect at a certain moment. Such as the MC not having a care in the world that he got his teacher pregnant, like that's as irrelevant as what foot you step on when you get up.
As a final thought I'd dare say this game is a hell of an incentive to learn the ropes at least enough to "fill the holes" (no pun intended...) and really turn it into one of those games you would never want to remove from your computer no matter what. As it is, it's not quite that good yet, but almost there.