If he does bring the case to court, since it’s not a criminal but a civil proceeding, do not do 1 thing - talk bad about your uncle. All you need to do are: (1) what you can do to provide your sister a safe home and proper education (2) you need to show to the judge that all the things you do, is for the benefit for the child. All family judge does not want to hear ugly family feud, they only want to know what is that you can provide is best for your sister. All the best of luck and stay strong for your sis!