DNA test

My partner and I have two children together,he has a daughter from a previous relationship. He wasn't put on the birth certificate but has been there for both the mother and daughter for the last 9 years both emotionally and financially..
After we had our children we separated for a few years and during that time he would always be aloud to see his daughter when he asked.
We have been back together for almost a year and the bitter ex just can't seem to get it in her head that we are going to stay together. No matter how hard she tries to come between us we won't be letting her get to us.. Our only problem is she is keeping his daughter away from us and tries to bribe us to being aloud to see her.

All we want is a chance to be able to spend time with her so we can bond with her and her brothers can get to know their sister. We also want to have a little as possible to do with the mother as she is very nasty,and constantly abuses us,drives past our house yelling and screaming as us.

Problem is we don't know where to start. She refuses to let us get a DNA test to prove he is the father (we have no doubt that he isn't the father) Do we call legal aide first and see how we go about getting a DNA test or do we call child support or Centrelink first.
We have proof that he has bee fathering her her whole life. But when we bring up the DNA test and mediation ect she turns around and says she isn't yours. She admitted that she told the Centrelink social worker that she doesn't know who the father is.. But is happy to take child support and a hell of a lot more from us..

Sorry for the rant we are just so lost about going to right way about this. Has anyone else been in a similar situation? If so what did you do?

Thanks mums ?