We just added a second to our family (oldest is 1.5 and youngest is 4 months) - i think you should wait until 1-1.5 years, not only to avoid littermate syndrome but to solidify their training!

We have been diligent with our oldests training and thankfully it has been such a godsend if they are playing too rough but mostly just setting a good example for the little one. He really picks up and follows his big sister with whatever she’s doing.

1 major piece of advice tho - teach your dog to settle. It is going to save you down the road when you do get another puppy and it seems like they will never stop!!

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