Quoting Chef Mommy:" What are some really cute ways to tell a 5 year old girl that she is going to have another sibling? She's ... [snip!] ... brother. But now that she is 5 I really want some great ideas to let her know she's going to be a big sister again! Thanks!"
I told my 4 year old that she was going to be a big sister, then I gave her the job to tell my dad (who I was scared to tell) and she had a blast, and still does, telling everyone she's going to be a big sister!
Maybe show her an ultrasound pic and ask her if she knows what it is?