How do you say Young Mistress in Japanese is it another word beside Ojou-Sama?

If by mistress you mean lover you can say 'wakai aijin', but since you don't I'd say I can't think of another Word for that, unless ojou-san. Owaka or waka-sama are used to refer to a young heir or young master of the house, but gender is not specified so it can be used for both male and female. If you are calling out to someone, onee-san also would be a polite way of saying 'young lady'. 'Joshi' is also used after female names, in formal situation like calling someone Lady, Miss, Ms etc.