In the 1800s and early 1900s only men smoked. Tobacco companies recognized that women outnumber men, so they started a targeted marketing campaign to attract women to smoke too. By 1947 60% of all Americans were smokers, including women and children.
Recently the porn industry recognized that mostly men watched porn, so they started a targeted marketing campaign to attract women viewers. Women fantasize about a lot of things, but incest is one of the top things on the list. Daddy's little girl porn, brought women into adult book stores. Today 60% of all American watch porn, including women and children.
Women rule the marketplace today. Men spend most of their money on important things like mortgages, insurance, car loans, food and drinks, with only a little of their income left over to spend on things like porn. Women don't have to worry about important things like housing, they can easily find a man to support them, to have a roof over their head all they have to do is ask... so women who work, have more disposable income to spend on less important things like porn. Shoes, and clothes are important, because they get that rent paid.
So it may appear that incest porn is popular, because the porn industry is pushing it hard... you can't even avoid it if you wanted to, but keep in mind that the porn industry has an ulterior motive for what they are doing.
Think of YOUR target audience. If you're makiing a game for young men, do you really think they want to see a young girl banging her old man, of course not. If you're making incest games, your target audience is women, and the game story should focus on her perspective. There's boy mom porn too, which is boy focused, but boys don't have income to be patrons. Family incest covers a lot of ground, but you face the problem of NTR haters. Women want all the family fucking, but NTR haters are gonna cry, if dad doesn't have a tiny penis and receding hair, which women aren't going to appreciate.
So ask yourself
WHO am I making this game for? You can please some of the people some of the time, but you cannot please everyone all the time. Make the game for your target audience, if you want the big bucks that women have to spend on porns, you're going to have unhappy male players. If you make a male focused game, you're going to have SJW hate, but who gives a shit about that. There isn't an adult game out there that everyone likes. You have to choose a niche of the market, and stay on that path... pissing off your fans is career suicide. Make two or 3 games for different audiences, but don't mix them up.