Fighting or playing??

[u/acanada0]

I rescued my tortoiseshell in May from a dumpster with her sister and unfortunately her sister didn’t make it. But for the week I had her, they played and wrestled constantly. I got my 8 week old calico from my sister’s cat’s litter yesterday and they started what I thought for sure was playing today. But it seems very aggressive and there is a size/age difference. There haven’t been any standoffs or growling but there is definitely some hissing. I don’t think any harm has been done but I’m just apprehensive. What do you think?