New puppy and older puppy

We have just purchased a puppy who has just met his new one-year-old sister. They continue to fight and the puppy has become aggressive. How can we stop them fighting and stop the new puppy becoming so aggressive?
Thank you,
Laura, via e-mail.
This is a very common scenario when puppies first come into contact with adult dogs. The puppy has not learned social skills or fully understood the whole "pecking order" thing (or has forgotten what his mother taught him). In most cases, the adult dog will tolerate a certain amount and then growl or snap to tell the puppy enough is enough. Remember that your puppy is learning all his manners and how to behave appropriately with other dogs. Socialisation is a good thing. You only need to intervene if things are getting really violent or out of hand. It can be as simple as "sin binning" the puppy for five to 10 minutes if he gets out of control. I'd suggest that you see your vet so the dog's behaviour can be assessed (he probably needs vaccinations or other vet treatment) and you can discuss these issues. Enrolling him in puppy preschool to mix with others and learn some obedience wouldn't hurt either.

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