Hi wizard. Anne Boleyn and Katharine Howard were cousins. I think 2nd or maybe even 3rd cousins and at least one generation removed but they were related. Anne Boleyn's mother was a Howard. They were a huge and very powerful family. It was Anne's uncle (the Duke of Norfolk) who engineered her relationship with Henry and he did exactly the same again with Katharine Howard (another niece) a few years later. He was pretty much responsible for their deaths - they were both just pawns in his power struggles.
Sorry not to be more specific (I don't have any of my books on the period to hand).
As an aside the Howard family still hold the Norfolk seat - now called Fitzallen-Howard the present Duke is the premier Duke in England and they are still Roman Catholics.
Catherine of Aragon ( daughter of Ferdinand V first king of Spain) was married to Arthur prince of Wales, Henry"s brother, so she was his sister-in-law. When Arthur died Henry married Catherine.