I do understand the “lazy”, I saw them around me when I was a kid too.

It’s really simple: everyone is born curious, that’s how we learn to walk and talk, or the difference between sucking a thumb and chewing on sweets. Then, some get encouraged to keep being curious, while others get forced into strict rules and punished for any displays of curiosity, until they stop trying.

I almost got pushed over the line, was labeled a “bad kid” more than once for not following the rules, and just by a lucky twist of destiny, they couldn’t push me more into a mold. That lead to later rebellion, family conflicts, further attempts that completely backfired, and I ended up being one of the few weird kids in class who kept being curious. I still kind of envy some of the others, who instead of getting stifled, got truly encouraged in their curiosity, they’ve ended up much better later in life.

My point being, these people are not lazy, or like to stagnate, or like being ignorant. I’m sorry for your brother, it sounds like his curiosity has been beaten out of him into oblivion. You say your mother has encouraged it, and at the same time that he keeps getting grounded; that’s how the process works. If any time one tries to get out of the mold they get punished, while getting encouraged to stay safely inside, it leads to just giving up, at least until they get to live on their own, which prevents one from achieving their full potential.

As for the answering “I don’t know” to every question, I’ve seen it as a defense mechanism in people who got criticized/punished for every answer in an arbitrary manner, so they’d rather automatically pretend not to know, than risk thinking and saying the wrong thing. Not sure how you could help them, nothing of what I tried did work in that case. Hope you have more luck.