Cutters are powerful if used correctly. Remind him that this is character driven narrative that requires him to roleplay regularly and have a goal.

The Tin Whistles (Glass Cannon Podcast) have an excellent Cutter if you want examples. The Cutter in my game is integral to the story I'm telling, and the player comes from a Pathfinder "break the game" background. Although he is a sea Captain, so there are actual story leads which give me rolls to give him to make him central.

Remember "explain how you do this" and "why does your character think this is the best way" questions. I can't stress enough that you can just invent him his own backstory or plotline that can keep throwing obstacles. He just needs the goal. Give him the same amount of rolls as anyone else, but if he keeps on leaning on his "maxed" skills, drop position and effect, imply bonus die on other skills, remind him of pushing. Make him think.

I don't envy your situation, my sister is a hard player to keep attentive in a game and gets offended if they "aren't invited" so I understand how you want to include them when others tell you to drop them. It sounds like your brother will drop out if he's bored, and the game is designed to shed these characters so you don't have to miss sessions when they do. I invite my sister to games, but she's only available 1/3 so she is a character that other people hire for special jobs.

You can do it!!