Our girl was 3.5 when DS was born. She was our baby and allowed on the couch, so after we got the all clear at the 12 week scan, she was not allowed on the couch anymore. She has always known "go to your bed" (in the lounge room) and so this was her "safe spot", especially when DS started moving we then taught him he was not allowed near her bed as this is where she went to get away from him pestering her.
Lots of people thought she would get her nose put out when DS was born as she was our baby but we knew she would be fine!! DS and her are now great buddies, DS helps feed her every night and to get her out of the laundry in the morning etc.

Good luck x