Night Terrors???

You've only had this little girl for 2 days...give her a chance to settle in and get to know you. Right now, you're basically a stranger...and many small children have a survival instinct that makes them afraid of strangers. Has she been removed from her mother's home for very long? If not, she's very likely to be grieving for her. Small children love their mothers, even if they abuse or neglect them. I would focus on making her feel safe and secure in her new home. I wouldn't worry too much about her crying waking up the new baby. I would worry more about about her fears and her loss at this point. On the other hand, it's entirely possible that she's just having night terrors. Most kids this age experience them, and you're right, they often don't respond to vocal reassurance...because they're still in a sleeping state, kind of like sleep-walking.