Incidentally, Guyur = Tail.
This comic gives me the feels, so let's focus on something super technical!
Only one uryuom was involved in this project, which means only one uyruom donated their DNA. While Guyur may have provided the eggs that were used for Grace and her brothers, he only contributed DNA to the creation of Vlad and Grace. As a result, among Grace and her siblings, only Vlad and Grace are blood relatives.
Don't get me wrong: They're still siblings, and still family. I'm talking purely from what DNA testing would have to say about it, and those tests are notoriously ignorant of the complexities of family beyond their data.