No, I think it's because of VISA / Mastercard. They have a list of what kind of porn you can sell.

The number one porn site on the internet does not take a credit card to access.

Don't they have an ad-free premium version you can pay for via credit card?

There probably isn't. But actual incest is risky and (often) illegal, therefore lots of porn sites disallow videos depicting it, therefore producers avoid it.