It's totally natural. I was once given this analagy. Imagine your husband coming home one day and telling you he was getting another wife. Switch all of the things you would say to a child, only use yourself in the place of the child.
"It'll be OUR wife. We'll love her together"
"I'll need you to help with this new wife. Teach her things and play with her"
"I'll have room for both my wives. I'll love both of you equally."
"I'm not replacing you, honey, I just wanted another wife to love as much as I love you"
No matter what you say, she'll be jealous. That doesn't mean stop trying. She will figure out she's not being replaced. It'll just take some time for her to get used to it.