My Daughter was 3, when I was pregnant with my 2nd child.
ALL during my 2nd pregnancy, I "Prepped" my daughter about it... about my being pregnant and the 'phases' I will go through. ie: my tummy will grow, I will get tired, I will have a hard time moving around, I will not be able to carry her when my tummy is huge etc.
I explained... about her baby brother in my tummy. That he is her baby brother, family, a sibling. We took photos all the time, of her and my tummy, she sang to him, talked to my tummy etc. She had already "bonded" with him, even while he was in my tummy. So that, by the time he came home from the hospital, she was ALREADY adapted, to her baby brother. And she adjusted very, well.
I also, explained to her, about what a baby is: that it cries, that it will wake, the Mommy nurses him etc. But it is Mommy's job. Not her's nor does she have to worry.
I made 'my' pregnancy, about PREPPING her. That whole 9 months.
It helps a great deal.
I also explained that I know she is a child herself and that, she just be herself. She does not have to act all grown-up. Just because she is the oldest. She can tell me anything she wants. I am THERE for her. To always tell me her feelings. That her things are HER things... I don't force her to share it. That I know, her things are special. SHE can decide, if/when she wants to share. That is an important thing for a young child. They need to know... that their things and 'their' routines...are theirs.
Just explain things to your child.
So they know what to expect.