Infinity,
My heart was absolutely SINKING as I read your first two paragraphs, and was SO relieved that you were able to save Mia....WHEW !
But, when you have one like Nina, who is h*ll bent on being the Alpha, you don't have too many choices. It is possible that your are not giving her enough recognition, that is, IF you are ok with her being the Alpha ?
What I mean by that is...
If you want to go along with Nina's roll, then you should start feeding Nina first, petting her first, greeting her first, letting her lead the Pack on walks and just anything else you can think of, which will
reassure her of her Alpha roll.
I KNOW how tough that's going to be, actually more on you than it will be on Mia, but having you as Pack Leader, "approve" of Nina's position, may settle things down between the two. Just to be sure NOT to let Nina see you petting Mia or paying too much attention to her for a while. You can take her aside out of site of Nina, and catch up on the lovin, but DON'T let Nina see that.
Your other choice if this doesn't settle down, is re-homing one of them...sorry.
Note:
I'm wondering how many reading this, can/will appreciate the distinction between Pack Leader, and Alpha Dog, and that there can be both ?
Note2:
Also, You need to get Nina aside, train her or practice training stuff... sit, roll over whatever, but your MUST enforce your dominance over her.
You MUST be the Pack Leader in her mind (a much stronger position) or the next time you attempt to intervene, you may get bit seriously.
Remember, "please" doesn't work in these situations, your word must be law !