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 (the younger son of the elderly Mr. Bunny) has reached adulthood alongside Peter Rabbit, and he has formed a romantic bond with his first-cousin Flopsy (Peter's sister).