It could even be possible that 'Sage' is a youkai species; which would explain similarities between Okina and Yukari.
"Sage" is most certainly a title. Especially if you consider the fact that the Sages' races is either mentionned explicitely or can be guessed with some certainty : Yukari is a youkai, Kasen is a certain something, Okina is a god. Even Eirin, who is not one of the sages of Gensokyo, is known to be a Sage from the Moon despite her being a heavenly goddess being fairly obvious. I'd say it's some kind of title received by people who are not only wise, smart and strong to a certain measure, but also have contributed quite a fair deal towards the well being of whatever place they are from. Of course Yukari and Okina would be pretty similar in various way if they were to earn the same title.
Also, to add on what has already been said and in consideration of the thread's title, It'd be pretty weird for Yukari and Okina to be sisters considering them being of different species (god & youkai) : that doesn't seem to bode quite well with the metaphysics surrounding these species : one does youkai stuff cuz of youkainess, the other essentially
cuz she's a god and it's part of what being a god is . But dunno, maybe we can pretend that they are "sisters" in a sisterhood (... Sagehood ?) sense so, hey...
Anyway, this might just be my impression, but I kinda had the feeling that in the past there has been this general consensus that all of Gensokyo's Sages founders were essentially Youkai : with what HSiFS brought to the table, it turns out that is not the case : the possibility of the Sages being of different races and obviously coming from different background kinda adds stuff to the whole Sages dynamics and could even raise some question regarding their relationship. Dunno how WaHH's Kasen-being-a-Sage chapter came to other people, but them talking about "sides" and apparently not often being in contact nor knowing what the "others" are doing seems really suspicious to me as far as the Sages' relationship goes.
Like, Yukari and Okina would necessarily have common traits that derives from what they are capable of and what they did to earn the same title, but considering very big differences in the metaphysics that surrounds them, them being sisters seems VERY unlikely to me. They are very similars in some ways, but also very different in others. Also, I am highly doubtfull of Okina's source material being said to have some kind of siblings or something.