Edit: but then I find stuff like this that at least offer a plausible mechanism of action (messing with nuclear transport proteins and preventing viral rna from crossing) idk

There is a plausible mechanism of action. That doesn't make it a good treatment or even an effective treatment in humans. "Plausible mechanisms of action" are basically what scientists look for to find new medical stuff to research, it's better than using a dartboard. Then you start researching how something works "In Vitro" which means if it works in a test tube. There are lots of things that can kill Covid in a test-tube; ivermectin, bleach, UV light, fire, etc. Then after that there's a long process for testing it in living things (In Vivo). It has to not only be effective at curing Covid, it has to do so at doses low enough that it doesn't harm the human.

TLDR: Tell your brother that taking ivermectin to cure covid is like drinking bleach, getting the concentration high enough to actually matter is going to kill you and taking an ineffective dose will just make you sick. If you think your brother will start drink bleach, tell him something else.

Edit: I'm not seeing the word placebo a lot in the first link. What exactly are they comparing against?