Suppose that two parents have a child named Sam before divorcing. Sam's father then marries another woman and has a daughter, Daisy, while Sam's mother marries another man and has a son, Matthew.
Because Sam shares a parent with each of them, Matthew and Daisy are both his half siblings. However, Matthew and Daisy are not siblings in any way, for they do not share any parents nor do they share stepparents.
Could Matthew and Daisy legally get married and have children if they are living in a state where incest is illegal? Would this count as incest at all?
I would also like to know if there is a term for Matthew and Daisy's relation to each other. (Quarter siblings? Some type of cousin?)