Why Nag preferred the brother of Chaitu?

One of the most interesting projects of tollywood to go to floors in forthcoming days is that Dr.ANR, Nagarjuna and Naga Chaitanya's multi starrer flick 'Hum'. Earlier, it was expected that ANR would play the grandfather, Nagarjuna the son and Chaitu as the grandson of this movie. However, later, their roles have been changed into ANR as a father, Nagarjuna the elder brother of Chaitu and Chaitu the younger brother of Nag. 

'Why has Nagarjuna preferred to do the brother of Chaitu rather being his father?", is the interesting question generated in the minds of movie folks. Nagajuna is of the opinion that he has very good following among ladies even today and they still expect romantic entertainers from him. That's why he is currently doing a romantic entertainer which titled 'Sarileru Neekevvaroo' and is willing to do only elder brother's role to Chaitu in 'Hum'.