I think I will have to do what SWHouston says and let her be the alpha dog. I'm thinking back and remembering I had started to feed Mia first as a way of telling Nina I didn't want her to be dominant over Mia. I have also been putting Nina behind because of her aggressiveness. This has only made her furious as she once used to be the alpha dwarf (lol) and now I have her apart from the doggie pack. She does
only get aggressive towards Mia.
I will be spaying Mia in a month and later Nina because I'm currently having a really tight budget. I hope this does help to calm them both down. Although I really wish it was my boy, Bambi, the alpha dog. He is the most "rational" buddy in the house (lol).
It is not an idea I love because Mia is such a kind girl and so naive that I feel bad having to reassure she is below Nina.