Barking

Question
We have chocolate and tan dog and bitch from the same litter. They are 24 weeks old and have been socialized with other dogs at puppy classes. They are wonderful with children and adults and obey sit, come, stay commands etc. However the dog barks and howls very loudly, whilst wagging his tail, at other dogs. He invites attack and frightens his sister. Any advice welcome

Answer
Not sure from your description exactly what is going on.  Has he been neutered?  If you get them altered, it will take about 3 - 4 weeks for you to see a difference but he may be showing male dominance - if he is trying to attack them, he needs to be neutered asap and his sister spayed so that they do not mate each other too.  He may need more socializing after neutering - not sure how he behaved in classes so it is hard to say how well he got along.  You an also try some calming meds for a little while and make sure these barks and episodes are not coming from a need to protect his people or anxiety.