Its NORMAL for your daughter to react that way.... let her express herself.
ALSO... 'allow' her to "not" have to share everything with baby.... really.
Kids, NEED to know, that 'their' things are special and not all things will want to be shared. Don't scold for this.
I never, made my daughter, who was about that age, when I had my 2nd child... 'share' everything. I let her know... that she has HER things too. And she can put things out or not, for her baby brother.
Also, I explained to my daughter, about baby development, in ways SHE could understand. ie: that a baby cries, wakes up during the night, that Mommy breastfeeds.... but it is MOMMY's 'job' to do that... and she does not have to 'worry'. IF things bother her, it is OKAY... she CAN tell me. I don't expect her.... to be 'perfect' nor the 'oldest' child... just because of baby.
I also took tons of photos of her WITH my growing tummy... and she loved that.
We also made up special handshakes and head nods... that the 2 of us could do... and that 'bonded' us and made her feel special...
I incorporated my daughter into my pregnancy... not making it 'my' pregnancy.... I even took her to all my prenatal appointments, which my Doctor encouraged... and he even taught her how to use the Doppler heart monitor on my tummy. She LOVED that....
she 'loved' her brother... even before he was born....
Have a ROUTINE just for her... once baby comes home. An 'eldest' child needs that.
Also.... do not put a ton of 'expectations" on her, just because she is now the 'eldest' child.... that can be too much 'stress' on a young child. AND... she is still a child. Only 'expect' age-appropriate.... reactions from her... not more than she can do or understand.
If she acts out... comfort her. Don't just 'expect' perfection in an eldest child. Never use the eldest child, as an 'example' for the younger sibling... that is TOO much stress for the eldest.
You NEED to talk about it with her.... and incorporate her INTO the baby coming home.... have her OWN routines... her OWN time.... her own adjustment to it all.... it is not only a baby coming home... but about her "Mommy" changing too.... and her family....
ALSO never 'compare' the 2 children... nor in front of her.
Make her important too....
not just a satellite, floating "around" everything that is going on.... so she knows HER place in the family.... still.
I ALWAYS spent 'more' time on my eldest child... even after baby came home.
And you WILL, have to juggle the both of them....
My daughter, was your daughter's age, when I had my 2nd child... which was also a boy, just like you.
This is what I did with my daughter... to help her adjust.
She loved her baby brother.... and they are close and 2 peas in a pod.
Remember, a child, does not automatically know 'how' to be a "Big Sister." They are only a young child themselves. The parents, don't even know 'how' to handle now 2 children either... right away. So, only 'expect' of your Eldest... what she can age-appropriately, do and understand.
ALWAYS let her know... she CAN express herself... good or bad or frustrations.... and she is allowed to. And you will be there for her.... even if just venting or feeling insecure.
I always told my Eldest "You are my FIRST baby... and always will be...." Even to this day, my daughter, the Eldest, LOVES hearing that....
I do not say that in front of my son, of course.
You ALSO have to explain to your daughter... what will happen when you are in the hospital, who will watch her, what will happen once baby comes home... etc. And you have to, spend time with her.... and Hubby will HAVE TO help too... and spend time with your Eldest. Have special gifts for her too.... not only for baby... not only everything revolving around the new baby.
AND when visitors come... make a point... of talking about your Eldest child.. and complementing her.... too.
ALSO, yes it will be unpredictable.
Its normal.
BUT ALSO... "allow" your Eldest... to have alone time too, if being around a baby, a crying baby, stresses her or makes her fussy. A child needs time away, for themselves, too. We as Moms get tired from it.... and a child too. They are young... and have needs for their own time 'off' too.
all the best,
Susan