In The Flopsy Bunnies, Benjamin Bunny (son of Old Mr. Bunny) and Peter Rabbit are adults, and Benjamin has married his cousin Flopsy (one of Peter's sisters).
In The Flopsy Bunnies, Benjamin Bunny (son of Old Mr. Bunny) and Peter Rabbit are grown-ups, and Benjamin has wed his first cousin Flopsy (one of Peter's sisters).