How old is your yorkie and when was his last vet visit? Your sister-in-law should be able to give you this info. It's possible that it's stress. He's just probably nervous because of the new environment he is in. I suggest you contact the vet if the vomiting persist. Don't stress too much yourself. Your dog will be sensing it and would likely get more agitated.
I hope it helps. If not, you can always ring an emergency animal hospital and just ask them. They'd be able to assist you.
