It's absolutely possible. You just have to go through all the drudgery to get there. You have to have the characters acknowledge what makes incest wrong, you have to have them come to terms with their mutual feelings, and you have to make them both consent. Remember, people who play incest games are willing to accept that sometimes feelings are more important than taboos.
I think it's best, with incest, to make the aggressor be the sister/daughter/brother (in the case of female or gay protag). That clears your MC of any implication of pressuring the character into anything they aren't ready for, or haven't considered. It also makes your characters better for it, most npcs are not proactive, their passivity can sometimes make them come off as more bitchy or cold than necessary, and of course, like gold diggers. So, you manage to make your MC look more respectable and have a more engaging plot!
This doesn't work with mom or dad however, because of the perceived coercive nature of their authority. So you have to be really careful here to make things mutual at every level, while also not making either person come off as creepy. To be honest, I haven't been able to think up something where an M/S or F/D relationship works that doesn't do the separated at a young trope. Sisterly Lust, if you ignore the part about her job, actually does a good job of giving us the M/S progression. I actually enjoy that plot more than any other M/S plot I've played. But, as I stated, it has the separated at a young age trope. It's a lot harder to not make mom or dad out to be total creeps if they raised you.
I would suggest you come up with a reason for the incest. Is the game focused on it? If so, how? What causes your MC and the other characters to develop or realize their feelings? As I said in your other thread, a good one you can use is jealousy. A sister or cousin or even mother, can start realizing, after you get a girlfriend, that they miss their time with you, that they want to be the center of your world. Of course, you have to make their relationship a real thing though. You have to convince me that they had a great relationship. Maybe they went to the movies every week, but now that you're dating on that night you haven't gone in a while. Maybe you use the well trodden trope where a character claims they have a lover to their friends when they actually don't and then they beg you, and one thing leads to another. It's all possible, you just have to think on it.