sisters yogita rameshbhai nandwana , five , and anisha , three , and brother harsh , 18 months , live in india .
they are among world 's heaviest young children - weighing 5st 5 lbs , 7st 8 lbs and 2st 5 lbs respectively .
food eaten by three siblings - who have sister of average weight - in a week would feed two families in a month .
father rameshbhai nandwana is planning to sell his kidney to earn the money needed to see top specialists .