What do I expect in an incest game?
Actual incest, not perpetual blue balling!
And yeah, the situation is already touchy enough without involving rivals, etc. Sure, there's the drug/spell/mind control route to get the ball rolling, but getting things there on their own accord story wise without using those crutches is something that we don't see done well all that often.
And if you aren't a 'Shakespeare' level writer, don't pretend to be. Write to your strengths, and if you just want to 'get to the bangin' so to speak in your story, don't overcomplicate things.
Example:
One thing that we see in porn videos a lot, but I personally haven't seen much of in porn games, is the 'girlfriend invites your sister to join the two of you in a tryst' angle. She can talk up how good you are in bed to your sister over several days, and of course your girlfriend has that incest 'kink' that she wants to see/be involved in, so she slowly seduces the sister to the point where things happen...
And, just to make things more interesting, the MC has no idea this is about to happen, until he looks down and sees sister sucking his dick (she sneaks in after things get going, and the GF may have put a blindfold on him, etc.). He thinks about stopping it, but sis is doing such a good job... and the fucking commences from there.
Then, of course, the brother and sister avoid eye contact for the next couple of days, because neither wants to admit how much they really liked it. Eventually, though, their biological urges take over and they go hog wild...
The thing that makes this fun and different is that in this case the male MC isn't chasing his sister, and that can be a surprise plot twist that he doesn't see coming.
No mind control, just good clean fun!
I'm sure there are a couple of games that use this angle, but not that I've played anyways...
Another fun angle (from some porn movie):
The MC goes out for a night on the town with his buddies, and hooks up with some young gal. They have a great night, but it's a one night stand so they leave things there. The MC then falls for another, older gal that's closer to his age, and eventually those two elope and get married. He hasn't met her now fully grown kids yet, and has only seen their 'younger' pics, so when he meets her daughter, he finds out that her daughter is the one he slept with earlier... and of course the mom has another daughter... and he has his own kid (a son)... yeah this is step-incest, but still has lots of potential.
You can flip this, and have the son fuck some older cougar type gal, and then he finds out that his dad has married said cougar... you can combine these two variations as well, if you really want to get things going quickly...
More fun angles:
Son comes home to stay with Dad for the summer, and finds that he has some 20-something gal that he recently married/eloped with. So of course, the younger wife and the son are attracted to each other and things happen.
Mom gets invited to an orgy/swingers party... and her son is there but they don't realize it until they are in the middle of things...
Bro goes to a strip club. Sis appears onstage... (she's working her way through college). Or better yet, Sis is the stripper/escort that answers the call for a bachelor party that the Bro is the chosen best man. And of course, the bachelor of honor and his friends want to see kinky stuff between bro and sis... 'cuz hiz friends are all perverts, ya know, and they are willing to tip very generously if this happens!
Dad and his friend hire an escort over the phone to entertain them for the night. Guess who shows up? The Daughter... (maybe his friend 'arranged' this but isn't telling).
Really, there are so many 'plot devices' from porn that you can use to get things rolling. It's dovetailing them into your story that's the trick.
Myself, as long as the story is somewhat plausable, I'll buy it, at least for the purposes of an adult/porn game. I just don't like seieng many of the gals in these stories acting like stupid naive clueless young ladies that don't have brains in their heads... intelligent gals have sexual needs too!