10 month old puppy aggressive with my sister’s 3 years old dog

Hi everyone, i'm turning to you for advice on how to prevent my 10-month-old Westie from becoming a resource and people guarding dog. Since he came home, I've socialized him with different situations (sounds, people, other dogs, children), and I don't have any problems at home because he's the only pet. My sister lives in a house with a yard and has two intact dogs, one 5 years old and the other 3 years old. Since he was little, I've taken my puppy to my sister's house so he could get used to sharing with them. I thought they were adapted to living together when we visited. The problem arises when the 3-year-old dog (a small Yorkshire terrier) comes to my apartment and my puppy becomes a resource and people guarder. He's not yet neutered, as I was told we could perform the operation after he turns one. For now, the few times he comes (when my sister is traveling) we keep them separated by a fence, but I'd like for them to be able to live together in the future without fighting. I'm looking for a behavioral therapist to help me handle this in the best way possible, but I'd like to know if you've been through similar situations and how you've addressed them. Please excuse any typos; English isn't my native language.