My MIL does the same thing to my husband. Two of her children refuse to visit because of their father's abusive behavior growing up and she frequently complains about it to my husband. I think she wants him to tell her that his siblings are exaggerating, that it wasn't that bad. Sometimes it's clear she wants him to pass along messages to the completely estranged sibling. Other times, it's like she wants to drive a wedge between her children. Maybe your father is worried you feel the same way as your brother and brings these things up so you can reassure him you don't? Whatever his reason is, it really is annoying!