William IV's niece was not Queen Victoria.
In some cultures and family traditions, it is common to refer to cousins with one or more removals to a newer generation using some form of the word niece or nephew
 By regular sound changes, the native, inherited English form of the name should be *Wilhelm as well (although the name is not actually attested in the history of English, and the Anglo-Saxon Chronicle refers to William the Conqueror as Willelm)

