That's tough. A good book to help kids understand the changes with having a new sibling is 'Za Za's Baby Brother' by Lucy Cousins. It has the older sibling a bit frustrated with all the attention the baby gets and how mum & dads are so busy now. It ends with Za Za getting special time with mum and dad once the baby was in bed.
I found my 2 yo DD settled down after about 12 weeks and it all became normal again.
Good luck!