Are your stepdaughter's children grandchildren

Your stepdaughter's children are someone'sgrandchildren.

If your stepdaughter became your stepdaughter at a young age, so that you treat her as a daughter and she thinks of you as a parent, then both you and she are likely to want her children to be treated as your grandchildren.

If she was an adult when she became your stepdaughter, whether you treat her children as your grandchildren will depend on your relationship with her, and the desires of her and your spouse.