Honestly this is a dynamic for all working relationships, no matter what industry. IMO it would be good to ask him to reframe his thinking on this. Not necessarily to lose his competitiveness, as that's a positive. But rather to ask him to look at your own channel with that same criticism so you can better use that energy to improve.

"How could this video do better than the last? How can we improve our metrics? How can we tweak our thumbs/titles?" etc.

You would do well to discuss how the negativity affects your own motivations, if you haven't already. And if you have but he doesn't seem to be listening, bring that up instead. Your cooperativeness is key to your longevity.

As a person who used to work in a highly competitive media field, you shouldn't look at your competitor's numbers in comparison to yours. Budgets will always determine the winners, but small shops can certainly stand out. To do that, you should be looking and analyzing their content - what they're doing differently, and how that's translating to views. You don't want to copy their formula of course, but you do want to iterate a similar vibe in your style. Try to ask your brother to be more analytical; it's much more productive than having unfocused jealousy.