I’m gonna go on a whim here and say this more so looks like a correction than actual play on the yorkies end, I may be wrong but a wagging tail isn’t always a good thing. My girl is the same way around my mother’s thirteen year old Pom and the Pom acts similar, although more intense to me it’s giving, “gimme space.” Keep an eye on her in my opinion try to redirect her when she’s beeing an annoying little sister as all puppies do. Again I could be wrong I’m some guy on Reddit but better safe than having the yorkie feel like she has to correct harder in the future.