Your daughter is trying to get back to a time when the baby was not born and she had all the love and attention of the household. And she didn’t have to share any of it. So she wishes the baby wasn’t there and pretends he isn’t.
Give her reasons to feel wonderful now that she is big. Tell her about the qualities she has which only big girls can have. Tell her that you just love having a big girl in the house. This will help her feel proud of being big. She will understand that she is special and that mummy loves her big girl.
Help her acknowledge the baby and to interact with him. Let her touch the baby, play with the baby, sit alongside the baby, and talk to him. Let her hold the baby (while sitting on a bed to avoid the fear of any mishap) Avoid saying “Don’t touch the baby.” Build trust in your child. Do this under your supervision-but never make the older child feel your anxiety when she is touching the newborn. However, you can’t allow the sibling to carry the baby until he reaches school age.
While your children are interacting say things like “How are my two babies doing? Are they having fun?” Or better still “How is my big girl taking care of her little brother. She is happy to have you sit next to him”.