How can a teenage girl suffer from anorexia or bulimia for years and her family not notice and or do something about it?

It's hard to see something that you don't want to see.

because us Ana's are sneaky buggers! ;)

I was bulimic, off and on, for 7 years and the only person who ever knew was my ex bf. I was also anorexic when I was younger and no one ever knew. I was very thin, but not thin enough for anyone to think that there was something wrong. It depends on the severity of the condition, how well the person hides it, and how much the people around her pay attention.

As long as no one ends up in the hospital, the family will be in denial.

Yes. People with anorexia often times love to cook and be around food.. they just don't eat it. They can take food with them so you think they're gonna eat it and then throw it away or claim to have already eaten.
People with bulimia actually tend to be of a normal weight so it's hard to tell by looking at them and of course they can be witnessed eating.

I was bulimic for three years and the only person who knew was my best friend. Only because I told him. My family never knew anything about it. They still don't to this day. Some people are just blind to a lot of things.

For the same reason that a family can have a kid on meth or crack for years and his/her family not notice or do something about it. Denial like one person said, not giving a crap or not noticing, just plain not realizing there is a problem. Also, they may not know how to deal with it and instead of seeking help or they are embarrased and choose to keep it a family secret. No matter what the reason its irrisponsible (too lazy to look up the correct spelling lol) for the family to sit back and let it happen.