You can explain to her that she's like a baby and will be mostly eating and sleeping, but you'll need her help in looking in on the puppy occassionally, not bothering it, picking it up, etc... but just making sure it's still breathing and also let her pick out a special toy from her to the puppy. I have an 11 year old son and he said to not let her get too involved, because then she'd think she was more in charge than she should be, and she might bother her more. But, if you didn't include her somehow, she might resent the puppy and maybe hurt her.
Hopefully, you can come to a Happy understanding between the 3 of you. Where will your Step-Daughter be during the day? Will you be home or your husband? If not, can you take the puppy with you in it's crate. It's a great way to start crate training - you could use that as your reason for taking her.
