I'm going to tell my bro that the age gap will be his doom
Are you guys alright? What age gap are you referring to?
He's dating a girl older than him.
If he wants to date an older woman, that's his choice! It's not up to you to dictate how your siblings live their romantic lives. Let him be happy!
That's why I'm going to break his spirit.
Do you really need to see him miserable? He's your brother, I think it's best you try to support him and his choices because it's his life and preferences. How sad would he be if you prevented him from chasing happiness?