What a difficult situation for all of you. I agree with Marda that something's going on with your little girl, or even a combination of things; sibling anxieties, bed change, possible separation anxiety and bad dreams are among the most obvious.
There are subtler potentials for sleep disturbance with which you might want to experiment:
Now that days are so short, she may not be getting enough bright daylight to help set her day/night sleep schedule. The blue end of the color spectrum, as in natural daylight, affects the production of brain chemicals like melatonin that regulate wakefulness and sleep. Get her outside as much as possible during the day, as weather allows. Let in as much daylight as possible – for some toddlers, even napping in daylight works better than napping in complete darkness. The brain handles naps somewhat differently than nighttime sleep, and complete darkness for naps may actually be working against her sleeping at night.
Likewise, avoid sources of blue light, TV and computer screens, within the last two hours before bedtime. Nightlights are necessary for emotional comfort sometimes, but they may actually interfere with sleep, particularly if the light that emanates is cool bluish or greenish.
Ask your pediatrician about using melatonin at night. Some think this is safe in tiny doses (perhaps only 1 mg) or even beneficial, considering how important sleep is to overall health. I've seen reports in this site of kids on the autism spectrum sleeping better with melatonin. It may be acceptable for other children, as well.
Be sure she gets lots of physical exercise during the day. Dancing, pillow fights, tumbling around the rug, trips to a park, whatever it takes. A well-exercised body relaxes and sleeps better.
In case of teething or other discomforts, you might try a small dose of tylenol before bed. If it helps, then something physical is going on.
Finally, be aware that allergies to food and sensitivities to common chemicals can increase anxiety and mood problems, and make sleep less possible. I suffer from pretty severe chemical sensitivities, and can go for days on end with only a couple of hours sleep per night. I just can't stay asleep when I've been exposed to perfume, or air fresheners, or scented laundry products, or many common household cleaners, or even the smell that comes off of new plastics.
I've watched kids in group testing situations go from calm to crazy when a tiny drop of a dilute chemical is squirted under the tongue. And a large British study confirms that common food colors and preservatives do a number on sensitive kids. Try getting your home environment, and especially your children's rooms, as clean and non-toxic as possible. You can google terms like "healthy home" or "non-toxic home" for lots of tips and ideas.
Good luck.