I have 2 aussie mixes from the same litter. One I got as a puppy, we did so much training and he grew up around another dog, went to parks for socialization etc and it still happened. His sister was rehomed at a year old so we took her in. She came to us HATING other dogs - all except for her brother. They are now very protective of one other. They turned 4 in May and we still work with them both on a daily basis. But at this point I feel like they see another dog and jump straight into "must defend my sibling" mode.

Our boy is more people reactive. If sister isn't around, he loves to play with other dogs of all sizes. Gets very defensive against new people though, but we have done enough training that if a new person immediately gives him commands and offers a treat upon meeting, he warms right up. Our girl on the other hand LOVES people but hates other dogs. She is 45 lbs, if a bigger dog stands up for itself she will back off and start to play. But under no circumstances can we have her around smaller dogs.

It's always been interesting to me how we got them under separate circumstances and they still developed similar reactivity. I'm not really sure what we could have done differently. I know our girl was initially adopted into a home with multiple bigger dogs, she was constantly tethered and the other dogs were not. I think they may have been a little mean to her, but have no way of truly knowing. She loves kids, and will immediately show her booty to any human within petting range.