My DD was exactly 24 months and 1 day when DS1 arrived by planned section. She had also never been a great sleeper, by 12 months she slept through sometimes but usually woke once or twice she was never as bad as DC3 by miles, but that's another story and got worse in the months leading up to the birth of her 1st brother, til she was needing me in the room for hours to settle and waking frequently - she was very switched and verbal on and absolutely understood far more than I usually hear other parents crediting not quite 2 year olds with (she wasked me when she was 22 months if she could take "Little-Little" out of my tummy for a bit to cuddle and put him back in, she also asked me, just before her sleep went to pot, who would be his "actual" Mummy and Daddy - I had been child minding for most of her life and she was used to sharing me in the day but the mindees going home in the evening - it hit me as a shock, but then after thought was not surprising, that she was assuming I was somehow "looking after" the baby in my tummy and he would not be living with us 24/7!!
I spent the last 3 months of my pregnancy sleeping on her floor for all or part of most nights (we had also had a permanent international home move 3 months before DC2 was born and I felt very guilty for all the changes and upheaval).
Important part of the story is:
2 days after I brought her little brother home from hospital she started sleeping thorough, and bar the usual exceptions of bad dreams/ specific worries/ illness she has done ever since :)
I did nothing specific, I think she had been subconsciously incredibly anxious about the changes her little brother would bring, and once he was home she was incredibly relieved, and could finally sleep!
I hope that happens for you too Life - all I can advise is keep talking to her and reassuring her about how much you love her, and when the baby comes home keep the focus on her as much as possible under the circumstances (in an everyday way not an OTT one), and hopefully the sleep will resolve itself when she sees that her word hasn't fallen apart with the arrival of her sibling! :)
Good luck!