If my mom doesnt have custody of me and i just left my dads to go with her would i get in trouble with the law?

ok i kno i have asked a question somewhat like this and i explained the situation. well now my mom doesnt wanna do it because she doesnt want the law involved. well my fiance wants me to come live with him and his gma til we can get our own place because im treated well there. his gma loves me like one of her own grandaughters. and we had everything planned out but my fiance siad that if we did that without my dad knowing he could put my fiance in jail along with his family for helping. im 17 almost 18 and my fiance is 18 but i cant stand bein in this house any longer im tired of my step mom puttin her hands on me and gettin in my face and puttin me down. there is alot of things that my dad and her do that would get them in trouble. my fiances gma said i could come live there but she would feel safer if we were married which we dont have a problem with. the only thing is could my mom sign for it if she dont have full custody? i live in indiana if anyone could plz help me. everyone i kno wants me out of this house because 1 its kind of a bad place. 2 they treat me like im their slave 3 they put me down all the time 4 my stepmom constantly says im a piece of **** and . and i dont want to take a chance on her puttin her hands on me again and breakin my things. so if someone could please help me i would greatly apprieciate it