I also have my newborn which she is 6days old and my son he is 3 years old eversince I was at the hospital and he went visit the two days I stayed there at first he gave a sad face and started not behaving well while visiting like trying to call attention as well as ever since I got home he does not behave good and he is making me yell at him all the times I still love him and show him I love him too but he does things that make me get mad and I Start yelling sorry ladies I try to involve him on everything but he seems to be jealous and everything he does he thinks is a game...he knows when he is doing something that's not good but does it anyways.. Its a little tough I really don't want to keep yelling I'll rather do that instead of spenking him..time out does not work with him at all..
Get her involved in as many ways as you can, like deciding between two outfits on what baby should wear, passing diapers to you or blankets. When baby is asleep, spoil your oldest a little. Give her a treat you normally wouldn't and watch her favorite TV show or movie with her. My oldest turned 2 two days after my youngest was born and it was so nice to finally be able to play with him actively and not just watchband encourage his play. I've started taking my oldest to indoor play places to let him be a kid and run around in a safe environment and he loves it.
I'm in the same boat my baby is 2wks old and my daughter is also we 20 months. Its getting a little better. I just let her help with baby as much as I can. I try and keep her involved as much as possible. She still not herself but it's getting better. Good luck mommy!