Is there a proverb for "Don't ask a mother how her child is, due to her motherly bond she would be most inclined to remark he is the best"

Is there a idiom/proverb for "Don't ask a mother how her child is, due to her motherly bond she would be most prone/inclined to remark he is the best in the world" even when he is a brat in eyes of others (outsiders).

Probably not a common saying, but I think the idea you want to express is conveyed by the following proverb:

“Never ask a barber if he thinks you need a haircut” (business proverb)

• The barber is in the business of cutting hair; he doesn’t make money if you don’t get a haircut. If you ask a sales specialist if you need to buy his particular product, he’ll always say that you do.

(www.barrypopik.com)