I'm going to comfort my brother rather than taking my girlfriend out on a date.
Why does your brother need comforting?
I just want to spend time with my brother. I don't need to take her out just because I date her
While I find it noble that you're there for your brother to comfort him, it's actually quite rude for you to stand up your girlfriend specially if you have already made plans with her. Try to show up to your date and spend more time with your brother next time when you do not have any more plans with other people.
What the fuck? I don't need a lecture from a stranger.
I was telling you for your own good. If you do that to your gf she'll have to re-evaluate the relationship that could lead her resenting you. If you still want to spend time with your brother you better let your gf know.