Here is an excerpt from my country's Family Code:
No marriage may be contracted between:
persons of whom at least one person is already in another registered marriage;
close relatives (relatives in the direct ascending and descending line (parents and children, grandparents and grandchildren), full and half brothers and sisters (having a common father or mother);
adoptive parents and adopted persons;
persons of whom at least one person has been recognized by a court as incapable due to a mental disorder.
I.e. half-siblings and siblings are equalized (the same incest). On the other hand, incest is not punishable in our country, except in cases when incest was connected with physical and/or sexual violence.
And so - a brother and sister are in a sexual relationship - no one cares. The only condition is that both of them must have reached the age of consent (16 years old), otherwise one of them can be prosecuted for sex with minors. Let's say the brother is 16 and the sister is 15 or 14.
That answers your question. Yes, this is also incest in terms of the law.