My sister is getting her own dog tommorow how will my new puppy react?

well your puppy wont LOVE your sisters dog I'd expect some barking. Carry your puppy when your sister brings over her new dog... then after a few seconds put them down and let them sniff each other. if there is a lot of barking just have your sis carry hers and you carry yours then just walk closer together and let em' sniff that way (while being carried). they'll get along some time.