I feel that my step-daughter enjoy's hurting my daughter

My stepdaughter is my partner'd daughter she now 9 yrs old her mother had gone out drinking while she was six months pg
My partner and I now have 2 daughter together 5 yrs old and 2 yrs old I caught my stepdaughter shuve a ball in my oldest daughter mouth, get ready to slap her, push her off the chair
My oldest daughter has told me she is scared of her sister she love her big siter when my step-daughter isn't here my oldest daughter is always asking for her my step-daughter now Im starting to notice that my uogest daughter is getting scraches

Note: My step-daugher has seen abouse with her mom and her step-dad

What do I to make sure of my daughter's safty

[margypops]

There could be a jealousy issue going on if your step daughter sees the other two getting more attention, perhaps your partner could get involved more with his daughter and give her one to one time so she feels important, Focus on her positive side and praise her when she does something good.She should have bounderies set as it is not permitted for her to hurt the other child, is she like this at school or is it only at home. When it comes to punishments it is better when her Father takes over, and you dont become the only punisher. Good Luck I appreciate its not easy, sometimes they feel left out and more care and affection will do it.