Can I request a paternity test

I've always believed that my younger daughter was not mine. I believe my younger brother fathered her with my ex-wife. I've been divorced since 2001 & paying child support for 2 children, ages 4 and 8 at the time of the divorce (now 14 & 17). My ex-wife cut off all communications with me in 2006 after applying for CS enforcement in the state she lives (WI); I have had zero parenting time with my children since 01/2006 and my ex-wife has been poisoning both children against me since.

My older daughter who is almost 18 is definitely mine; the younger looks more and more like my younger brother every day, and my ex-wife is suspected of having an affair during our marriage - with both of my younger brothers. I am in the middle of a long child support modification case with my ex-wife - the first in our ten years of divorce. Everything has been thrown on the table, and I would like to have a paternity test done.

I am listed on my younger daughters birth certificate as the father, and my ex-wife and I were married when the child was born. Divorce was filed in Illinois; Wisconsin has enforced the order since 2006. I see that the state of Wisconsin's child support agency will not test for paternity since I am listed on the birth certificate as the father.

What are my rights? Also - if I can have a paternity test done, what are the chances that my DNA differs enough from my younger brother to show he is the father if it comes down to it?