To me it sounds like he's projecting his beliefs on to her. In her brain it must be ok, but you were obviously brought up different with different results. If she wants to get hit thats on her, even if you try to save her she will fall back to the same comfortable situation. Its on her to change.
Its hard to convince kids candy is bad for them and will destroy their teeth. They have to somehow see if for themselves, either through example, first hand or second hand experience.
You may want a better life for her but if "12" is always their then she's accustom to that lifestyle or doesn't know of other options which is also possible. It might be self confidence issue which is really tough to cure.
You speak as if your projecting a sisterly label to her and act accordingly. I can understand its hard to see people you "know" struggle but this sounds like a drama loop.