Oh my goodness a baby in the mix too, you have your work cut out Sweetie.
First off congratulations on what you have done so far, you clearly know your stuff and what is going on now is something you couldn't have avoided in my opinion, it's just one of those 'tricky periods' that happens for all LO's.
I have no experience of 'night terrors' but I do know many BW that do, and it sounds like they could be in the mix from what you describe. Here's a link for you to read through:
Nightmares and night terrors.I also am a Mother of an only child, so not the best person to advise on the effect of other LO's arriving on the scene, but I do know from my moderating here at BW that it can also effect siblings, so you are right not to discount that Hun. If you don't already do so maybe go overboard verbally during the day about her being your 'special girl' to confirm her special place in the family. I think often when a LO knows they are loved, it seems unnecessary but it can't do any harm to give that little bit more you know?
I have had some success with audio stories for DS when he has had these types of problems. Even if she is not having trouble going to sleep, it may be that she is feeling anxious about things that they are transferring to sleep, and that is why audio stories can help a child feel in a better place when they go over, meaning the mind can be more rested and on a better track, IME anyway (in my experience).
WRT her waking early in the morning, in the first instance I would work on her staying in her room quietly until it is an acceptable time to wake up the rest of the family, which is something that gets worked on and discussed a lot here. So a gro-clock can help:
http://www.amazon.co.uk/The-Gro-Company-HJ008-Gro-Clock/dp/B002APJCNE/ref=sr_1_1?ie=UTF8&qid=1431938880&sr=8-1&keywords=gro-clockOr some parents use a light on a timer. If you are okay with her being awake initially and just teaching her to stay put you could allow her to look at some books until 'wake up' time. The idea is once she has it in her little head that no matter what she is staying in her room, she may start to resettle. It's not ideal for her to be awake and looking at books, but while she is still in bed the extra rest really does help, and of course you have the rest of the family to consider, so sometimes it's just the way it is and the best solution.
When the NW happen, I would try and keep the interaction to a minimum. Of course if she has had a bad dream/night terror you need to stay with her, but otherwise just use a whispered voice and tell her 'Sleepy time Darling' without entering into any conversation.
Let me know your thoughts Hun, and any other questions you may have.
Vicki.x.