Awww.. She's 8 years old, she probably really enjoys having a little sister - I know that my daughter who is also 8 would love it! I wonder why you feel threatened by her behavior? She is just loving her little sister and being a little mummy to her - that's great!
Just remember your daughter can only benefit from all the love she gets -- and having a big sister - this is something she will love more and more, so even though you are feeling a little left out, try and nurture this relationship - it's so great when children have good relationships with their siblings, so try and take the long view.
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I agree with your husband , she does not see the baby every day so it is natural and nice she wants to play with her, perhaps you are the one feeling left out, how about you join in pay your step daughter more attention, some games and gal talk, letDad have the Baby and you take her out ,its a normal reaction but its coming from you not her , try going out as a family and accept her more forget this word 'step' accept her as a daughter , ..good luck