"Cached thought!" doesn't refute the thought. But OP suggests that persuading his sister of something true was made more difficult by the her cached thoughts on the subject. I've definitely seen similar things happen before. If that is the case then I've probably done it before. I want to be convinced of true things if they are true. So if I have the time and energy it is advantageous to suppress cached thoughts and recompute my answers. Maybe this is only worth doing with certain interlocutors-- people with sufficient education, intelligence and thoughtfulness. Or maybe it is worth doing on a long car ride but not in a rapid-flowing conversation at a party.

you never actually, personally thought the thought that was cached (it was just handed to you by your environment), or you thought it a long time ago and you need to reconsider the issue.

Part of the problem is that thoughts aren't tagged with this information. Which is why it helps to recompute often and it is often easier to recompute when someone else is providing counter-arguments.