Is a girl born of Zina (fornication/adultery) considered a Mahram (unmarriageable kin) to a boy born of Zina? And are the daughters from a legitimate marriage after repentance also considered Mahram to him?

The child of adultery is not attributed to the lineage of the adulterer according to the majority of scholars. However, he is attributed to the lineage of his mother. Thus, he is a brother to all her daughters and a mahram to them. Indeed, everyone nursed by his mother is forbidden to him. Likewise, he is a maternal brother to her children from a valid marriage, so all her children are mahram to each other.