I am not sure what is going on w/ you..but I want to make sure you know...diabetics are not all overweight. It wouldn't hurt for you to have a check-up w/ your dr..have some bloodwork done to make sure..if it's been doing this since you were 12..haven't you had some bloodwork since then?
I think you need to go to a doctor. Diabetes isn't only something that overweight people get, I've seen plenty of people who were fit except for having diabetes. You might not have it, but since your symptoms are happening repeatedly over a span of time, it's not worth taking the chance. In some cases, when you get the treatment is important, so if it's really worrying you, make an appointment with a doctor. In just about any city, there are cheap clinics and things like that, if your family doesn't want to take it seriously.
You DO NOT have to be overweight to have diabetes!! With your family history, symptoms and excessive thirst, you definitely need to be tested for it. Get to your family physician and have a blood test.
While you may NOT have diabetes, there is also a high risk (due to family history) that you may and you do not want to mess around with high blood sugar - it can lead to many much more serious problems.
If you can't get your parents to take you to the doctor, talk to your school counselor or nurse and explain your symptoms and family history and ask them to intervene on your behalf.
Best of luck!