My son..is only about 2yo..at this moment..he eats anything..salad, cooked vege..just about anything..I believe it's because I let him try and eat whatever we eat.  I do hope he continues like this till he's bigger.

My friend's son who is about 4 is a picky eater..he only eats chicken rice, nuggets or fries.  When i asked my friend..she told me because, she herself don't eat certain things..and when he was young..he just didn't like vege..hence she never forced him.

As such, i realised the following:
1) good eating habits have to be developed at a young age
2) parents have to instill good eating habits or at least let them try different types of food.  This is also a reason why i said in the other post (Solids for babies), that once our babies turn 1yo..let them eat what the adults eat..not just porridge..so that they become accustomed to normal food and the different taste and textures.
3) Maybe..it's just in them..that they don't like (but which i doubt)

As for your brothers...i think you got to try other ways to entice them as they are alraedy big enough to separate the vege/fish (if you mix their food).