Which group generally does not inherit from a parent unless adopted?

Stepchildren generally do not inherit from a stepparent unless they are legally adopted. Inheritance follows the legal relationship established through bloodlines or adoption, and stepchildren do not have a legal claim to the estate of their stepparent unless that relationship is formalized through adoption.

Biosiblings, nonmarital children, and half-blood siblings typically have inheritance rights as they are either biological or recognized under the law as legitimate heirs. This is why those groups don't fit the context of the question regarding inheritance without the formal adoption process. Thus, stepchildren are the correct choice as they fall outside the typical legal framework of inheritance from stepparents unless adoption takes place.