Could the crimes against nature be a part of the incest charge? Since most people think incest is very wrong, they consider it warped and so they add that.
It's a separate law. "Crimes Against Nature" was the statute that banned "unnatural" sex acts, like oral and anal sex, and sex with animals. Now that sodomy is legal nationwide, the ban on oral and anal only applies to incest now.
"Persons Forbidden to Marry", which is what Chris is charged with, was the statute that banned interracial sex, as well as incest. This is penis-in-vagina sex -- i.e. the act itself was always legal, as opposed to sodomy, and the statute just restricted who you could do it with. After interracial marriage became legal nationwide (Loving v. Virginia), only the incest part remained.
So that's why there's two incest statutes -- because they were really sodomy and interracial sex statutes that happened to cover incest too, and now it's mostly just the incest parts that remain.
EDIT: Also, interestingly, while the interracial sex part was struck down in 1967, the sodomy bit wasn't struck down until 2013, even though SCOTUS had declared anti-sodomy laws unconstitutional in 2003.
I'm not so sure about the "crimes against nature" charge. I don't think anyone wants to go into that much detail with Chris' sexual exploits with Barb. It's enough to know that they fucked (therefore incest), not how they fucked (icky).
They cover different acts. If they only had oral/anal but NOT penis-in-vagina, then only the "Crimes Against Nature" statute would apply. If they only had penis-in-vagina, only the "Persons Forbidden to Marry" statute would apply.
If they just did "outercourse" SheCameForCWC then it's perfectly legal since neither of the incest statutes apply.
So it's impossible to charge someone with incest unless you *do* describe exactly what they did. (Not to mention, a detailed description would have to come up in evidence anyway if it went to trial.)
EDIT: Obviously "outercourse" would be illegal if Barb refused consent or was incapacitated, or if Chris were underage, but it still wouldn't count as incest. When it's consensual, however, it is completely legal in Virginia for your mom to give you a handy or for you to stick your toys from Adam and Eve into your mom.