My two are siblings, so don't have experience of introducing another cat, but I have never seen them hiss at each other. The only advice I can give is make sure they have enough space so Nina has somewhere to go if things get too much?
But you are right, there will always be a 'top cat' and maybe they are just determining who it is.
Even tho mine are brother and sister, I still give them separate treats as Denzel won't eat them in front of Dexter who is the Boss!