I have mixed feelings about this. You said "pair of girls," which ... I've seen so many new writers fail in writing about two girls working off each other as co-main characters. This is probably more my hang-up than yours, but good God, don't make them total air heads or boy-crazy. My eyes have bled enough from dumbass dialogue from those kinds of characters and I swear I will ream you out for it if you make me suffer through more. (For the record, even if you are a girl, my warning stands. Both genders have done this, though I can't figure out why.)
Now, the above paragraph would not exist if you had not said "preteen/teen romance." For me, that is a red flag even bigger than this one. I would not be worried about "boy crazy" dialogue, but "preteen romance" is normally just god-awful. I've seen incredibly rare occasions were it was okay, but I've never seen it from a new writer. Teen romance is slightly less iffy, because I've seen multiple stories that have done it well. That said, it can still go terribly wrong, and if you consider Twilight or anything similar to be an example of "teen romance" rather than a train-wreck, get out now.
A book portal, I sincerely like. I've thought of using similar devices. I will say, if you're going to put it in their family members home, I do expect a damn good explanation as to why their grandmother had it. Wars ... can be interesting, but I'm usually nonplussed over the reasons or the methods. If you're creative enough, though, it could be very compelling and interesting.
Actually, that last sentence basically applies to every part of your story. I'm mainly saying that cliches, lazy explanations, poorly fleshed-out characters, and badly portrayed relationships are things I abhor. If you can avoid those via being creative and sincerely caring about your game's content quality, this story will be fantastic.