Welcome to YT! Your new babies are so adorable!
As for your question, I don't have litter mates but my yorkie and my maltese are constantly like this. It's how they play but when I feel like it's going too far then I'll break it up. I'm not sure if they'll grow out of it but I sure hope so for my sanity! I'm so scared that my new little yorkie puppy will get hurt somehow but she's the one always initiating the play fight.
__________________  Alfie & Princess
|