My sister stopped believing.... :'(?

ok well i dont know how many christians are out there but if there are...will u plz help me? i guess i should tell u what the problem is. my younger sister is only 11 and she stopped believing in God. and i know there is a God, no matter what anyone tells me. it all started when me, my mom and sister were in the car. kayla (my sister) started telling my mom about school and how she wishes she could kill everyone in her cl. my mom told her to stop her bad attitude cuz it sounded like the devil wuz taking over her and making her evil or something. my mom asked her how she could have God and Jesus in her heart when she talks that way, and my sister said she doesn't believe in God and she doesnt wanna talk about him ever again. plz help me......i dont know what to do. my sister has a problem and i dont want her to stop believing in God. cuz he's great and she needs him in her life. AMEN!!!!!