S.P.
Hi S.
It is tough for the siblings when a new baby comes home. I do agree that you can't let discipline go, you should have the same rules, and enforce them the same way now. We use time out as our main "punishment."
But, as hard as it is taking care of the new one, just try to make sure your oldest is included in everything that he/she wants to be. Make room on your lap for him too, even when feeding. Ask him to hold the new diaper while you change the baby and give him plenty of opportunity to kiss and hug and cuddle with the new baby. Tell him what a good big brother he is. This too will pass. My oldest was just about the same age when his sister was born and he got bored with the new baby rather quickly. Now, when number 3 came along 15 months later, I had 2 clingy older guys to tend with. You will all get through this, I promise!