It's true that the Celestica people were the first to live in Hisui/Sinnoh and there are history and tradition that started from them, but according to Old Verse 5, they were already on the verge of disappearing* by the time the two clans arrived at Hisui/Sinnoh from across the sea. Thus the clans aren't direct descendant in the genealogical sense.

*) I have to note that the writer of the verse wrote in absolute that they disappeared, but I have reasons to believe that this may be an exaggeration/dramatic for the sake of brevity and emphasizing emotion. The reasons being they probably stayed around long enough to pass down some of their traditions in wardenship and Noble Pokemon to the clans, and we have two characters who likely descended from them and one of them is still young.