Oh my do I know what you mean. I thought after my second was born (now 6 weeks today) that I had forgotten everything about bringing home a little one. I have a 5 1/2 year old too. Well the baby had jaundice so we were at first at the doctor everyday to have her tested and I finally asked at the beginning what was going on. My doctor which is wonderful and a mom herself reminded me that babies have growth sperts at about 2 and 4 weeks. So they will start to get in a routine and then it changes so they can grow. I know you are getting little and speratic sleep but it won't last forever. Because of your other child do things as normal have the lights on, tv, talk, etc. But at night low lights and be quiet so they know the change of time. They will pick up on things soon. Also, I am not one for having the baby in the bed but my doctor also said that if you have the baby with you like I did in the hospital and feed and if you both can fall asleep together then that is good too.
My little one had things all messed up and then she seemed to be up all the time 24 hours just eating. I was at the end of my rope and just had to sleep. My friend who is a labor and delivery nurse and my doctor told me to have my husband or son feed her a couple ounces of a bottle if she would take it. Nipple confusion should not be an issue and so I did and was able to get a couple of hours while my husband took his shift at things. That is the best thing I did. I also found she loves the swing so I put her in there and let her either just look at things and listen to it play music and swing. I can get ready and feel good again and my son lets me know if she makes noise or is awake. Also, I can sleep on the sofa next to her and hear her make noise if I need to get her.
If you need someone to talk to :) Good luck!