when we first got our missy, we had a Tibetan terrierX. when Tink came over to sniff the pup the first time, missy grabbed her nose with her paw, the licked her in the eye and the look tinker gave us
, she was disgusted!
next day missy was running around the living room and tinker was lying non her bed (as usual- lazy bean!) but got up to go get a drink, she came back to find a brand new toy to cuddle in her bed... until it moved!!! oh, she was Not impressed with that little furball at first, but after a week, missy was Her puppy, not ours
.
now when it came time to add a new addition, after tink passed, we got little kitty (yorkieXchi) and missy was thrilled! they played for hours (with glucose breaks) before crashing on the bed in the corner curled up together. but this was only 7pm, and when missy realised kitty wasn't leaving for bed time she was livid! she didnt go near kitty again for about 5 days, and took a lot longer to learn to share toys and beds and treats (missy and tink had separate ones because of the size difference) and she vibrated in fustration at the idea of it; i kept being told by my mum that my puppy was pestering hers, and it may not work long term because missy was terrified- she wasn't, just a spoiled madam.
all yorkies work things out in very different ways, but 99.9% of the time, they do work it out on their own and learn to love the new baby sibling!
(for example, when my mum brought me home and told my sis i'd be sharing her room soon, she wanted a hamster instead! but she warmed up to me; after trying to swap me for biscuits a few times! lol- true story apparently!)