Futa incest porn, though it may sound strange, is a very hot genre of porn. It is not uncommon for this to be the first time you have ever heard of such a thing. It may also sound strange to others, so you will need to explain it carefully.
The concept of "incest" may not be familiar to them, so you will have to expand on that a little. In the context of porn, incest is when one partner in the act is biologically related to the other. This may be any relation, from parents and children to siblings to half-brothers and -sisters.
Now add this to futanari, and you have a very special kind of porn. A futanari is a person with a vagina, and a penis, and often, an unusual amount of breast.
So for this scenario, we have a brother and a sister. The brother has a crush on the sister, so he decides to try and seduce her.
He eventually gets her into bed, when she reveals her 20 inch monster cock and rapes his asshole with it.
This is futa incest porn.
Enjoy!