What is the Shar'i and legal status (bequest, inheritance, mutual obligations) of a person born outside of marriage then in a civil marriage to a Muslim father and a non-Muslim mother, as well as their half-siblings from subsequent marriages of their parents?

The jurists differed concerning the lineage of a child born out of wedlock if the mother was not in a marital bed (i.e., not married). The majority of scholars held that the child is attributed solely to its mother, and the rulings of motherhood, such as inheritance and custody, apply to it, with no connection between the child and the fornicator. Others, including Ibn Taymiyyah, held that it is permissible to attribute the child to the fornicator if he acknowledges paternity (istilḥāq), and all rulings, from both the father's and mother's sides, such as inheritance and the child's siblings being his siblings from the father's side, would then apply. The Mufti (definition) believes there is no harm in adopting the second opinion, given the clear benefit it offers in these times.