Is a son born of an illegitimate relationship considered a brother to his full siblings, and must his sisters wear hijab in front of him, and does he have a right to inheritance? And does the ruling change if the father later marries the mother of this son?

The child of zina (fornication/adultery) is not attributed to the fornicator/adulterer, nor does he become a mahram (unmarriageable kin) to the man's daughters. Rather, he is a stranger to them, and they must observe hijab (veiling/seclusion) from him. It is not permissible for him to marry any of them. He has no right to inherit from the fornicator/adulterer; rather, he inherits from his mother.