] I wonder what the church would have said if any of Mr. & Mrs. John Knightly's five children wished to marry one of Emma and Mr. Knightley's children (assuming they had some.)? Were double-first cousins regarded as a closer form of consanguinity?

When reading "Madame Letizia" by Stirling, I found out that after Napoleon had divorced Josephine, his family wanted him to marry one of his nieces(i believe one of Joseph's daughters).

I had always thought this was considered unacceptable, but the biography indicated that their was no impediment to this.