newmummy I agree with Pepper and others after her who suggested making a big deal of decorating the other bedroom on the same floor as a big girl room for your niece, and not emphasising the link to the new baby. Either do that soon (tie it in with starting school maybe - desk, bookshelves, as well as more grown up decor) or leave it until your baby is coming up to 6 months old, which gives you ages. That way it doesn't feel as though she's being pushed out.
You'll know that babies need to be in the parental bedroom til 6 months old anyway. If you get a poor sleeper it will be much easier to have the baby in the next room than down a corridor.
On the other hand...
We have a set up a bit like yours bedroom wise - 2 bedrooms directly opposite one another (though they're the smallest, one not quite a box but a definite single and one a small double) a large one down a corridor and a few steps, and another large one in the attic.
Dc1 and 2 had the 2 rooms opposite one another and we had the big one down a corridor and down a step, and that was fine when dc2 went into his own room at 6 months.
Then dc3 came along and when he needed a room we moved dc2 into our room (he was 4), we moved to the attic and dc3 was given dc2s room (the smallest). We weren't even on the same floor as baby dc3, because neither of the other DC were old enough to be totally safe doing stairs from attic in the middle of the night.
Years later dc1 and dc3 swapped.
It's been fine, never really thought twice about being closest to baby. In fact both times dc1 has been close to the babies rather than us being... The baby has also always had the smallest room, then swapping at peak toy play age...
So you could just decorate the other non attic room for your baby - it's 10 months til you'll need it anyway, and if your baby was dc2 and neice dc1 you probably wouldn't stress about room swapping.
Good luck whatever you do!