Surely they knew their brother’s name, so why did they call him “your son”?
It seems to be a regular human habit not to use the name of someone you dislike. Saul says at a time when he is angry with David, “Why has the son of Yishai not come?” (I Sam. 20:27).
Husbands and wives tend to follow this practice; the husband is annoyed with his child, so he says to his wife, “Look what your son has got up to!” The wife is angry with her daughter, so he says to her husband, “Did you give your daughter permission to stay out so late?”
Note how the Torah tells the rest of the story. Jacob wasn’t having any of this “your son” idea. He looked at the blood-soaked coat and said, “Yes, it is Joseph’s coat – a wild beast must have attacked him”.
He added, Tarof toraf Yosef – “Joseph has been torn to pieces”. There is an implication, Tarof toraf ‘yosef’ – “(the name) Joseph has been torn out (of the family)”.
Just as there is a strict rule against deleting the name of God, by extension one should not delete the name of any of God’s children.
If there is an exception it is only for an Amalek: someone who is inhuman and inhumane may have their name eradicated (Deut. 25:19). That explains why we allow people to attempt to drown out Haman’s name. It also explains why we refer to the Nazi tyrant with the words Yimmach sh’mo – “may his name be blotted out”.