I watched this animated movie when I was little, so some details may be wrong.
All I remember is that there was this quite depressed guy (whose family just moved into town, <this part could be wrong), he tended to ignore his little sister. One day she goes missing, and with a help of his friends the guy finds her in this digital-like world, which was created by a man who kidnapped the girl. And the reason he took her was because she reminded him of his daughter/niece who passed away at a young age.
If my memory doesn’t betray me, the guy’s sister had a gap between her front teeth, and her hair braided back into few sections (like this but her hair was a lot shorter :
This is all I can remember