Ok thanks, that's helpful. No magic bullet I'm afraid, just a few thoughts...
How long has she been eating this amount of solids for? Just wondering if her little tummy might be finding it a bit too much, or whether there might be anything that she's especially sensitive to? Milk ought to still be her primary source of calories at this age, although of course there's a wide range of solid intakes at this age and it might be that she's coping with it just fine - just something to consider, perhaps?
Don't be afraid of a much earlier BT on days where she takes a short pm nap - 13h nights, or even longer, are possible. I know it's counter-intuitive, but an OT LO will very often wake early after a too-short night, and then the cycle continues... EBT can help to break that cycle, but I know it can be scary

Are you doing strict set naps, or do you adjust the time of the second nap depending on how long the first nap was? Some LOs can do well with set naps at this age, so I don't want to suggest differently if you've already found yours to be one of those. But if not, you may find she does better if you pull the pm nap a little earlier when she does a shorter first nap - again, to avoid a build up of OT. Some LOs need the A time after a shorter nap decreased as well, but others need to keep the same A time. If she does a shorter nap and you keep the time of the pm nap constant (ie set naps), then the A time increases and many LOs would find that too much and do a short nap (eg 45mins). Do you think that might be happening here, some days?
About moving her into her brother's room - I'm afraid I have no advice, since we still have our DD in our room as we're too worried about the same thing

But I know many others have done this with no issues, I guess it partly depends on how light a sleeper your eldest is?