My dauther n law 4yrs old turn into bad behaviour?

i am married my husband has a 1 daugther to his ex gf, now we have 1 son and im pregnant. i love my dauther in law now she is 4next month, her behaviour is bad,i mean she's unhappy she wont listen of what her daddy say NO bcz she's doing nauthy if you stop her she wont listen and get her daddy upset and im not happy for her bcz she make it hard for herself and make me crazy inside the house noise.she always conplain anything. and always get sick because she dont eat much.i dont know why she do that.im tired bcz i have a lil boy and im pregnant and make me upset for her and im not happy anymore to do anything for her because of her behaviour.do you think im bad. i love her before because she is a good girl and now she no respect and turn into mad. i need help bcz she's w/ us evry weekend.