Unless that family member was under age at the time, I'm not seeing anything illegal here.
Penal code 285 PC is the California statute that makes incest a crime. Under this section, marriage or sexual relations between close relatives is prosecuted as a felony as punishable by up to 3 years in jail.
The types of relatives who may not legally marry or have sex with each other are:
Parents and children;
Grandparents and grandchildren, or anyone else with an ancestor-descendant relationship (like great-grandparents and their great-grandchildren);
Siblings;
Half-siblings; and
Uncles/aunts and nieces/nephews. 1 2
Unlike with many other California sex crimes, lack of consent is not an element of the crime. In other words, you can be guilty of this offense even if both parties to the sexual relationship consented.
https://www.shouselaw.com/ca/defense/penal-code/285/
