My pup was the same way with my grams dog when I brought him home. But I gave him one on one time with me for training about 10 mins a day around 4-5 times a day. It's important that everyone in the household came make him listen to commands and such but if he spends the bulk of his time with you he knows your his mommy. I wouldnt worry about him wanting to see your sisters dog all the time, puppies love to play with other puppies/dogs.
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