(Azusa having a sister when she said she was an only child in the other games, Miki's not awakened but that's also debatable).
Yumeko adressing Azusa as her elder sister is her way of showing admiration towards Azusa.
Nowhere in the game has it been stated that they're siblings, whether it's blood-related or not.
She calls Azusa Miura her "older sister" because of her admiration to her as her senpai.
Watch more animes/play more games with (potential) yuri couplings and/or girl-only school settings.
The way I see it, iM@S DS might or might not've happened if you look at it from a bigger perspective;
Assuming iM@S DS did in fact happen one should assume that the 360/SP version happened as well, but in the way that not every idol started out on the same date; I believe that the DS cast only debutted a few months later then some idols.
This'll fix the problem why certain idols are less popular/famous then others *coughChihayacoughwaytooexaggeratedcough*.
Also, let's assume that SP happened because of a certain duo being in the agency.
Again, because if we assume that DS happened and because we assumed that not every idol started out at the same time we'll assume that only 3 765Pro idols beat Takane, Hibiki and Miki at the IU.
When you start out in iM@S DS with either Ryo, Eri or Ai, every 765Pro idol are all atleast in gameplay week 20 or so; all 765Pro idols're atleast known and atleast Rank C-B'ish.
Eri, Ryo and Ai are all working towards the Idol Ultimate, as well as all other 765Pro idols that haven't won the IU or some big audition yet.
Because you play a Producer in every other game and because you only meet up with your idol once a week, there's enough room for the DS cast to co-exist in the same time-frame as the other games; you can't encounter the same idol on every week day.
This leaves enough room for the an idol to meet up with her Producer, and because the Producer never met any of the 876Pro idols you won't see/ hear them.
I'm not sure yet if I'll apply this theory to the multiple rookie producers theory or the 1 iDOLM@STER Producer theory but that's not important for this discussion.
Or in other words, the settings for starting out at iM@S DS in my opinion would be something in the lines of;
3 characters cleared story mode of SP
A couple of characters might or might not've cleared 'Free-Produce' or the 360 game yet.
The rest of the idols're in week 20+ in either Free Produce or the 360 game.
After above conditions're met the DS games happen thus giving enough leeway for the idols to coexist in the same world.
Tldr;
Cheese
