Hello,
I really need to know that if two people consent to have a incest relationship, is it legal or illegal? I can't believe it would be legal, but I need to know.

Incest is NOT legal in California.

California code 285 on adultery/incest is not well written, but it clearly holds to the standard by which a marriage would be considered valid. Please see the code on "Void Marriage" below.

California Penal (Criminal) Code 285.

Persons being within the degrees of consanguinity within which
marriages are declared by law to be incestuous and void, who
intermarry with each other, or who being 14 years of age or older,
commit fornication or adultery with each other, are punishable by
imprisonment in the state prison.

California Family Code - Part 2 Judicial Determination of Void or Voidable Marriage

2200-2201 VOID MARRIAGE
Marriages between parents and children, ancestors and
descendants of every degree, and between brothers and sisters of the
half as well as the whole blood, and between uncles and nieces or
aunts and nephews, are incestuous, and void from the beginning,
whether the relationship is legitimate or illegitimate.

This law defines "Incest" in the California code and gives clarity to the Criminal code.