Australian Shepherd Nipping

I have a 9-month-old Aussie who is nipping people. My extended family arrived in the afternoon and that evening he nipped one of my cousins. She was sitting on the floor petting him, he was just staring at her and all of the sudden he nipped her and left a pretty good mark.

The next day my other cousin was standing and petting him when she started to leave he nipped her in the leg and left a large mark. I hate this but I don't know what to do.

Neither of my cousins are young children but he still nipped them.

Please give me lots of tips! I really need to stop this and I don't want to keep him locked up when we have people here.

I need help! Thank you.

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