What can i do about my 15 month olds behavior?

She is testing limits all kids will do this. You always need to be consistent with what is ok and what is not. She will act up more now that there is a newborn in the house. She was probably used to getting all the attention and now she has to share that attention and that is hard for her to handle. Try to make special alone time with her without the baby around if at all possible to let her know that she is still very special to you.