I don't care about incest specifically, but I have to wonder where they're drawing the line and why. If the games are depicting consenting adults, and only adults can download them, I don't see the harm.
Did they target incest because it's "icky?" What if they decide other perfectly legal things are icky? Shouldn't legality be the line?
It's not illegal to depict incest, the same as it is not illegal to depict many types of acts that many would consider immoral or even acts that are explicitly illegal, like bestiality, necrophilia, or rape. There might be an argument for regulating content that depicts illegal acts on the grounds that it normalizes or potentially glorifies these acts, but that isn't likely the reason for this. I think it is mostly a corporate choice and partially a legal issue.
I am against this partially because it is arbitrary but mostly because I don't like a precedent set towards banning things on moral grounds.
Paypal and other payment processors are wellknown to set up arbitrary rules. They held the money of an 100% legal and fine erotica author hostage for months, for example. Incest games vanishing is just step 1 of their censorship.