Can a slightly obese indian teenage girl inherit big belly from her father or is it the fast foods she eats that determines her belly fat?

Yes and no: You can inherit a tendency toward gaining belly fat, especially if there is a history of diabetes in the family, but it will only happen if you eat too many calories and do not exercise enough.

obese: Her obesity may partially be from genetic predisposition, but it is also very likely that she is either eating fast food, too much food, too many carbs, sugars, junk food, soda, etc. If she ate a healthy diet and exercised regularly, she would likely gradually reduce her body fat.