I do think that this jealousy stems from insecurity and Moosey knowing that I am her owner, not Ziggy's. But I think that even if I were to own Ziggy, Moosey would still be a little meanie. I try to greet her first at the door and talk to her when she is meowing. I give her all the attention she wants, but she seems to be lashing out at Ziggy a lot more.
Of course, this may be due to the whole "Ziggy not being fixed" thing.
I just want to live in harmony, but I guess I don't like everyone I meet, so why would cats like every cat they meet?