hi there, it's so draining when they have slept and then don't again.
It's an age where they start to notice shadows, see shapes, stories can scare them etc.
Does she gave a night light or do you leave the light on outside her room,?
At this age we had a lot of this too and really did what your doing...sitting somewhere near while my DS1 fell asleep then few nights later sat outside the room or just pottered about upstairs until he went to sleep.
I would give her lots of cuddles, read nice easy going stories at bedtime and just be there for her, it will pass.
I know it's hard with another but you could always negotiate by saying you'll leave a small light on while you settle the other and that you'll come back and check her regularly x