Push her first

OP I would talk to her dad about this. but put urself in her shoes. she's a ten yr old that sees u at fault for what was her perfect family breaking up. she will be bitter about this. talk to her, go to group therapy with her father/ur husband. explain to her that this is her baby sister. my family got divorced several times and currently I don't like my new mom. so I can know that feeling of resentment for a young or older child. just realize she is a lost and confused child that was in the middle of her family, a place a child should be loved and be safe from all the bad things, ripped apart. be careful, it's still a threat and with a child not being able to tell right from wrong or the full extent of what's going on, I would be careful and have her father around. she obviously loves her father and would not do anything to disappoint him. good luck OP and congrats on the new baby :) I hope u can show this little girl that u can be apart of a new beginning to a happy family.

My dad was dating my mother when she got pregnant, and he had a son who was 11. My half-brother never threatened to hurt my mother (or me). So I'm going to go with: feeling like they're going to be replaced is normal. Threatening step-parents however isn't normal.

Beat her ASS

I hate my stepmom 0.0 but she has a son with my dad -.- so I'm stuck with her.

Wow. This is one of those "life's little annoyances" all right.

Run

I would do the same thing to my stepmom

Damn, that kid needs therapy, or a good booting. Make a point to bring it up as often as you can, for instance when on the stairs. "oh, YOU go first, we wouldn't want an unfortunate accident, now WOULD WE." It'll embarrass her if she regrets saying it. Also keep this one ready for the 21st birthday...

Don't go upstairs for the next 7 months