Wouldn’t it of been werid for Catherine of Aragon to marry her husbands brother?
Yes, is the simple answer!
Henry VIII did require a papal dispensation in order to marry Catherine of Aragon because she had been married to his brother, Prince Arthur. Marrying your brother's widow was seen as incest because of what was written about such a union in Leviticus but Catherine assured Henry, and everyone else, that her marriage to Arthur had never been consummated. On the grounds that the marriage had never been consummated, Henry really didn't need the dispensation but I guess that he wanted to cover his bases.
Here is what Leviticus says:-
Leviticus 18:16 "You shall not uncover the nakedness of your brother’s wife; it is your brother’s nakedness."
Leviticus 20: 21 "If a man takes his brother’s wife, it is impurity. He has uncovered his brother’s nakedness; they shall be childless."