Why is my stepmom so bossy ?

My stepmom acts like she can tell me what to do she just married my dad and she thinks she has the right to punish me when she dosent know me she has been grounding me an taking my stuff when I do little tings and she onily dose it when my dad isent around my real mom pased away and now I have no one to talk to an this lady is a ***** but why dose she think she has the right to punish me?