My ex husband passed away he owes me 3000 in back child support and medical. His sister is incharge of his will and assets and is refusing to pay me the back pay how can I get my back pay from her.

If there is an estate ( if he has anything to pay out) you will file a claim with the probate court

You go to Probate Court and file a claim against his estate. What happens next depends on the State and the Court Order(s) in effect at the time of his death.