Is discussing bad practices of monks a sin? I've heard in several dhamma deshana that discussing the bad practices practiced by monks is a sin. But I feel that if we don't discuss such with our family &amp; just continue giving, we won't have much shradda in our heart &amp; also we won't be able to inform it to the chief monks to be corrected. Is there any suthra that describes about this?
(Hatthi,gāmaka) Ugga Sutta

In the above Sutta householder, Ugga declares he does not discriminate how he treats the monks based on their attainments or qualities. This is his 6th quality. 

But if some monk is doing something wrong it might be worthwhile to report him to a head monk if there is a chance that proper action might be taken.

Ultimately what matters is the motivation. If you talk ill of a monk with intention breaking his good standing with other this is bad karma. If you do it for the betterment of the monk and the sasana then it is positive. In doing so you have to be tactful though so that it does not lead to unintended consequences.