Look, here is your statement:
"My sister has blue eyes and so does her husband. But her son has brown eyes and it's because one of her parents (my dad) had brown eyes."
Now, to have truely blue eyes, one must have two recessive blue allels --- bb.
Sister = bb
Bro-in-law = bb
You will only get blue eyes. Period.
In the case of your sister having true blue eyes, the brown color of the your and your sister's father's eyes is irrellevant, other than as a statistical oddity.
(The reason for this is that each sperm carries only 1/2 of the DNA set of the father, as does each egg, albeit for the mother.)
Now you mention your sister's father having brown eyes.
Brown can be expressed BB, Bb, or bB.
The circumstances indicate to me that your sister is Bb --- which generally results in brown eyes ---- but can result in Hazel.
Here your sister passed on the "B," which took the normal course of being dominant and resulted in a brown-eyed kid, instead of your sister's hazel.
Oh, and before you accuse people of "not knowing the most basic elements of science," you might want to read their profile.
Her eyes are blue. Never mind the recessive gene, the actual color when you look at them is blue. Sure there is the brown gene, but the color is blue. I am not going to call something blue hazel.
My main objection to your utterly offensive remarks were your sick imputations against my brother in law. You owe me and my family an apology for that. You had no right to throw out those sick Hillary-esque statements about my brother in law apparently needing to study science based on incorrect assumptions about my sister. He never HAD any incorrect assumptions. But you sure did. And you owe us all an apology for your offensive, despicable accusation which is an insult in the truest sense of the word.