well tanya is a good bahu to viranis because she appeared to them for 20 yrsbut it doesnt mean that nand cannot b a gud bahu
its just that nand hardly got any oppotunity to prove herself in all these yrs while tanya had enough of time to do her chores
by the time nand made her entry tanya had already made an impression
and it was damini who proved to b a better bahu cuz she was the one who took care of baa in tulsis abscence and tanya used to just stand and watch everything even the quarrel between karan and sahil
a bahu is the one who tries to unite a family
and when it was time for nandini to play her pat i think she always stood with tulsi and no wonder she got the keys of the family and the fav bahu award