accidental story about incest?

*Spoilers*
Dont read if youve not watched the movie.

He's been wanting to marry Gwendaline and then discovers who is at the end - which is Gwendalines first cousin.

The characters happily state Johns aunt and younger brother but never points out that he wants to marry his cousin.. is this an error that they story writer had done? Did they only realise the error once it was written and hoped no one would notice if they didnt mention the fact because the movie would take an unwanted sinister twist of incest?