Custody of a child

My younger is married since 6 years and she has one 4.5 yrs old daughter.After two years of marriage she got to know that he is a drunkard and as per Muslim law it is haram.Secondly he doesn't take care of expenses and all no proper home to leave.Every three four months he shifts as he doesn't give rent on time on top of that he enjoys his life fully and he is like that my wife and daughter is with their nani and mom so he doesn't take much responsibility of them...After seeing all this my sister is with us since a year with her daughter,he went in jail n number of times so many reasons..since two months he is coming to visit his daughter for five mins give some goodies and then leave..no education responsibility nothing..As my niece is at my place due to summer vacation so he got to knw that his daughter is with me since 2 weeks so he keeps on harrasing me and everyone in my family that I want my daughter back at Nani's place.Recently my sister joined a new job and she is earning for her daughter to complete her education and now suddenly this guy is dyeing to tak take care of daughter and wants them back but my sister isn't ready..He kept on giving dhamki to my family specially my brother k he will murder him aur he will kidnap hi wife and daughter once the daughter is back from my place..so how to go about it in this case?We want custody because we know he is not capable enough nor his family