I believe that the true meaning of incest is that the two persons having sex must be related by blood.
Everything else is simply two people banging who happen to belong in the same extended family by someone's marriage.
While that is broadly true, the law generally recognizes as incest any sexual relationship between a child and step-parent as well, even if both are adults. No blood there.
I found this answer on a general web forum, about what is incest:
"sexual intercourse between persons so closely related that they are forbidden by law to marry"
"This typically includes sexual activity between people in a consanguineous relationship (blood relations), and sometimes those related by affinity, such as individuals of the same household, step relatives, those related by adoption or marriage, or members of the same clan or lineage."