I'd say Nina is the real queen and Arika is Rena's daughter; the evidence has been given before. Mashiro is a random child that was picked up and MADE the queen, and is the only one with experience being in command.
I'm expecting her to rise to the occasion and appear like a true ruler after... um... stuff happens and they're in position to take windbloom back.
Either that, or this little theory:
Nina: Random orphan who got knocked on the head and helped by a worrying Rena during a diplomatic mission with the queen, and who was eventually saved by Sergei.
Arika: Genetic experiment, a cross between Bird!Alyssa, an Ant and the black beast of ARGHHH--
Mashiro: Temporally and dimentionally misplaced daughter of Shampoo and Nabiki from Ranma 1/2.
...what do you mean, they're both girls and can't have kids?! Then what are all those pics of Natsuki and Shizuru's kids, hn!?
As for who the queen and Rena's daughters are:
Rena's daughter: Tomoe. After all, she's from windbloom... </conspiracy theorist>
The real queen: It's obvious.
I mean, come on. Everything points to it. And don't tell me you haven't thought about it. No one else would fit as well in the queen's robes, especially if they look like the one Mashiro wears on occasions.
The real queen is obviously Sakomizu.
...