There are different perceptions of what incest is. For some people it’s the taboo of crossing traditional relationship boundaries. A lot of those people would lump in-laws in too.
For others it’s only about shared DNA. A lot of those people would exclude in-laws.
For what it’s worth, the actual definition of incest is explicitly about the shared DNA. Nobody perceives the world through definitions though.
At this point, with modern contraception, it doesn't matter much about DNA, but don't fuck people that can't go elsewhere for thanksgivng. Once you stop, then things become weird even if you're not technicality related by blood.
Don't fuck your family means don't fuck your family.
But my mind recalls an excellent essay by an anon a long time ago explaining why in-laws, step-siblings, etc became so frequent in porn. Something about an easy plot setup and a few other things.