Same here,our daughter is five and my hubby and i are having a boy May 19th,shes been the only child for four years up till not and when we told her we were having her little brother she started showing out,misbehaving, acting like a baby.We sat down and had a talk with her about it.I call her my big girl and tell her ill need her help with her brother,she cant act like a baby,she has to be big sis so she can teach him things.It helps to know tht she will be involved and not excluded.Knowing that shes big sis and she can look after him gave her some confidence.It helped us so u can try that.I take her with me to my appointments and let her hear her brothers heart beat,or touch my belly and feel him move and kick.She loves it:) and it gives her a chance to bond withhim before he's born.
I have 2 little girls that are 5 and 7 and due with a baby boy April 28th. My girls act similar but yea its stressful, I tell them that they are gunna be big sisters and they don't want their little brother to act naughty. They like going shopping with me for the baby. Things like AMDG said find little hinhs for them to do. I'm here I u wanna talk. Good luck. :)
Maybe they feel like they're being replaced and they're acting out. Maybe it would help to sit down and tell them how much you care about them and how this baby will be their little brother and you will need their help to take care of him. Maybe that reassurance that they are still important will be helpful. Maybe give them a job to do, make birthday cards for baby, help pick stuff out for him, help decorate his room, and they will feel excited. Just a thought.