I never personally experienced it, because my little sister came 18months after me so if I did get concerned, I don't remember.
BUT my sis was 5 when bro #1 was born, and she acted as your DD is acting. My mum was pretty sick when pregnant with my bro (got ross river virus, had bro premmie at 34 weeks, and they both spent eight weeks in hosp. after an emergency c/s). Mum has always put it down to three things: fear of losing her place in the family, fear that this baby was going to take mum away from here (as in quality time, and also the possiblity of something fatal), and she also became resentful of the baby because, in her mind, it was the babies fault mum got sick, the babies fault mum had to stay in bed and not play with her etc.
Unfortunately, my parents were the opposite of you, and honestly we weren't a big part of their life during that time (kind of understandable now, but not to a 5 and 7 yo), so my sister (now 19)still has extreme resentment towards my bro (who is now 14, and has CP).
I think, you're doing the right things. I do think that its important that there are still aspects of her life that are completely hers and not permeated by the baby, if that makes sense. Even though, once the baby is born, it will become your whole world, remember that it doesn't necessarily become her whole world.
I'm not a mum (yet!), so I'm just writing from a daughter/sister perspective, but I hope this helps you all the same.
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