I doubt she's a master. She obviously dropped the martial arts to go modelling, leaving the fighting arts for Sekai to inherit. I reckon she must still be doing plenty of physical exercises to keep her body suitable for modelling, so it's possible she's still repeating some familiar moves. It doesn't seem to me like her personality would be as good for martials arts as Sekai's anyway, and it also seems like Sekai's image of her sister might not be entirely correct. Probably he hasn't updated his image for years and remembers his big sister from back when he was very young and Mirai already seemed like an asskicker with the Jigen Haoh.
It didn't look too good about the Sekai+Fumina. He's denser than iridium and while Fumina was embarrassed, she hasn't really shown any romantic interests whatsoever during the show. At least her mom was immediately ready to pair her with Sekai. Seems like Gundam Build mothers are the best...