Child support??please advise!!?

question for my sister?? she is a single mother of two children which in her past life chose the wrong person to deal with! so now she has been seperated from her ex for over 10yrs and due to domestic which he served time for..he is now bettering his life and they are gong threw the paper work to be actually divorced now..he is now working and making a good amount of money and for the last 3yrs of working my sister didnt really get on him for the child support now that she is he is fighting to pay her anything and tryng to take her to court to avoid paying... my sister has a very good job and does make about double what he makes could there be a chance she wont get nothing??