Use GRO Index. George William Sykes mmn Brook was born Dewsbury 1854. He was one of the many brothers of Edward Brook Sykes mmn Brook.
Technically he could have married her in the registry office. Prior to the 1921 law referred to by Heths the Anglican Church refused to perform marriages which were banned by the consanguinity rules of the Book of Common Prayer. That did not make it illegal, just very difficult as long as the only place you could marry was in the Anglican Church.
Dave